mta-bor298001_FinalHydraulicMemo.pdf

Prepared for:

WESTERN FEDERAL LANDS HIGHWAY DIVISION

HYDRAULIC DESIGN MEMORANDUM

Sun River Bridge Replacement

MT FLAP BOR 2980(1)

IDIQ Contract No. 69056721D000008

Task Order No. 69056723F00008N

September 2023

Prepared by:

ROBERT PECCIA &

ASSOCIATES

Helena, Montana

Sun River Bridge Replacement, MT FLAP BOR 2980(1)

Hydraulic Design Memorandum

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TABLE OF CONTENTS

Abbreviations/Acronyms ........................................................................ ii

1.0 Project Background

2.0 This Memorandum

3.0 Drainage Characteristics

4.0 Existing Siphon

5.0 Existing Bridge Assessment

6.0 River Assessment

7.0 Hydrology

7.1 Drainage Basin Delineation

7.2 Hydrologic Methods and Analysis

7.2.1 Design Flood Frequency and Freeboard Requirements

7.2.2 USGS Stream Gage Data Analysis and Flow Summary

8.0 Bridge Analysis and Recommendations

8.1 Existing Condition HEC-RAS Model and Calibration

8.2 Proposed Bridge HEC-RAS Model

8.3 Proposed Bridge Scour

8.4 Proposed Bridge Abutment Riprap

8.5 Other Concerns At The Proposed Bridge

Tables

Table 1: Existing Siphon Table 2: Existing Streambed Gradation (Visually Estimated) Table 3: Sun River Peak Flows Table 4: Sun River Bridge Hydraulic Comparison Table 5: Proposed Bridge Scour

Figures

Figure 1: Project Vicinity Map Figure 2: Existing Bridge Elevation View – Looking Upstream Figure 3: Existing Bridge Pier (Pier 3) Figure 4: Existing Bridge Center Pier, (Pier 2) Figure 5: Existing West Abutment, and Downstream Bank Figure 6: Existing Upstream Channel Figure 7: Existing Downstream Channel Figure 8: Sun River Site Overview Figure 9: Stream Gage at Existing Bridge Center Pier Figure 10: Exposed Bedrock at Proposed Bridge Location

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Appendices

Appendix A: Existing Siphon and Bridge Photographs

Appendix B: Sun River Hydrology Analysis

Appendix C: Existing Sun River Bridge Inspection Report

Appendix D: Existing Bridge HEC-RAS Model Input Data

Appendix E: Existing Bridge HEC-RAS Analysis

Appendix F: Proposed Bridge HEC-RAS Model Input Data

Appendix G: Proposed Bridge HEC-RAS Analysis

ABBREVIATIONS/ACRONYMS

cfs cubic feet per second

FHWA Federal Highway Administration

FEMA Federal Emergency Management Agency

GPS Global Positioning System fps feet per second

HEC-RAS Hydrologic Engineering Center River Analysis System

MSE Mechanically Stabilized Earth

NEPA National Environmental Policy Act

OHW Ordinary High Water

PDDM Project Development and Design Manual

Q# #-year design flow

RCP Reinforced Concrete Pipe

RPA Robert Peccia and Associates

RS River Station

USGS United States Geological Survey

WSE Water Surface Elevation

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1.0 PROJECT BACKGROUND

The Federal Highway Administration (FHWA) is intending to complete the design and National Environmental Policy Act (NEPA) documentation for replacement of an existing bridge over the Sun River as part of the Sun River Bridge project.

The proposed bridge replacement project is located along Pishkun Road at the eastern border of the Lewis and Clark National Forest. In this area the Sun River marks the border of Lewis and Clark County and Teton County, Montana. The proposed project comprises approximately 0.25-miles of new approach roadway construction (with approximately 0.2 miles being on the east side of the bridge) and replacement of the existing Sun River Bridge with a new three-span concrete bridge structure approximately 270 feet downstream. The existing bridge is a two-span steel truss structure that is beyond its service life and has restricted load limits.

The project begins near the start of the asphalt pavement section of the Sun Canyon Road at the east boundary of the Lewis and Clark National Forest. It extends 0.25 miles to the northeast across the Sun River and connects with the existing alignment of Pishkun Road. The project requires new roadway construction including clearing and grubbing, excavation, grading, drainage, aggregate surface course, guardrail, revegetation, right of way documentation, and utility relocation. The project also requires new bridge construction including drilled shafts, concrete work, mechanically stabilized earth (MSE) wall construction, and pier and abutment construction. Depending on the funding available, demolition of the existing bridge might also be included in this project.

The proposed project is located approximately 17 miles northwest of the town of Augusta, Montana.

Approximate Global Positioning System (GPS) coordinates for the center of the project are 47°37'06"N, 112°41'32"W. A project vicinity map is shown in Figure 1.

Figure 1: Project Vicinity Map

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2.0 THIS MEMORANDUM

The purpose of this memorandum is to document the hydraulic conditions associated with the existing drainage and bridge structure, as well as the hydraulic analysis and design recommendations for the new bridge structure over the Sun River along the proposed project corridor. This memorandum will discuss drainage characteristics, hydrologic and hydraulic assessments, and a summary of bridge hydraulic recommendations. Potential floodplain encroachments are also discussed.

3.0 DRAINAGE CHARACTERISTICS

The project area lies at the eastern front of the Rocky Mountains. The topography of the area is characterized as mountainous with steep timber and glaciated slopes. The project area falls within the Northwest hydrologic region as described in the United States Geologic Survey (USGS) Scientific Investigations Report 2015-5019. Timbered slopes with peaks extending above tree line, and glaciated timbered valleys dominate the area. The Sun River is the only river crossing along the project corridor.

This river includes approximately 610 square miles of drainage area contiguous within the Lewis and Clark National Forest. Approximately 65 miles east of the project bridge site, the Sun River discharges into the Missouri River at the city of Great Falls, Montana.

The drainage basin upstream of the proposed bridge site is heavily regulated for irrigation purposes by two dam structures constructed and owned by the Bureau of Reclamation and operated by the Greenfields Irrigation District. Furthest upstream from the existing bridge, approximately 4.31 river miles upstream, sits the Gibson Dam. The Gibson Dam is a 199-foot-tall concrete structure capable of storing 96,477 acre-ft of water. The stored water can be metered out for irrigation needs. Downstream of the Gibson Dam, approximately 0.86 river miles upstream of existing bridge, sits the Diversion Dam. The Diversion Dam is a 132-foot-tall concrete structure capable of diverting 1,400 cubic feet per second (cfs) to a nearby irrigation canal. This canal parallels the Sun River on the west side to the existing bridge. At the existing bridge, the canal flows into a 10-foot diameter Reinforced Concrete Pipe (RCP) siphon that conveys water east under the Sun River just downstream of the existing bridge.

4.0 EXISTING SIPHON

A 10-foot diameter RCP siphon parallels the existing bridge no more than 37 feet downstream. This siphon was constructed in 1944. The siphon conveys water diverted by the upstream Diversion Dam east underneath the Sun River to the Pishkun Canal. It is a pivotal structure for Greenfields Irrigation District operations. A structural inspection of the existing siphon will be performed in concurrence with the Sun River Bridge Replacement Project. Other than the inspection, the siphon will not be disturbed by project activities.

A summary of the existing siphon data is listed in Table 1 and photos of existing siphon are included in Appendix A.

Table 1: Existing Siphon

Location Existing Siphon

Field Notes Photo # Diameter (ft) Material Length (ft) Slope % Condition Service Life

37 feet Downstream of Existing Bridge

Conc. 698 N/A Unknown Unknown Custom inlet and outlet structures 1,2

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5.0 EXISTING BRIDGE ASSESSMENT

The existing Sun River Bridge was constructed in 1916. It is noted as federal bridge structure MTA-SR- 001 and is a 2 span, 222-foot-long steel truss structure. An edited elevation view labeling truss member nodes from the 1916 as-built plans is shown in Figure 2.

Figure 2: Existing Bridge Elevation View – Looking Upstream

The deck width is approximately 14 feet, and the structure is generally perpendicular to the river channel.

The low beam elevation is governed by the two center spans and was surveyed at 4,388 feet. This elevation is approximately 23 feet above the thalweg of the river. The structure’s depth is approximately 13.9 feet.

The bridge substructure consists of 3 concrete piers. Piers 1 and 3 are located at the extreme ends of the bridge structure and are mostly buried. The typical arrangement of Piers 1 and 3 is displayed in Figure 3.

Figure 3: Existing Bridge Pier (Pier 3)

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Pier 2 (center pier) is constructed with a triangular concrete nose pointing upstream and a square end facing downstream. An active stream gage is affixed to the rear of the pier. Pier 2 is shown in Figure 4.

Figure 4: Existing Bridge Center Pier, (Pier 2)

The bridge end abutments are not protected by riprap. Both abutments are perched above the channel on the canyon walls and do not appear to encroach into the floodway. The west abutment ties into the canyon wall at an elevation of 4,386 feet, about 14.3 feet above the Ordinary High Water (OHW) and can be seen in Figure 5. The east abutment ties into the canyon wall at an elevation of 4,391 feet, about 19.3 feet above OHW. Channel banks appear to be stable with exposed bedrock and/or heavily vegetated slopes with large trees and bushes. Photos of the existing bridge are included in Appendix A.

There is no Federal Emergency Management Agency (FEMA) delineated floodplain associated with the Lewis and Clark County side of the Sun River in this area. FEMA has delineated Teton County’s lower portion of the river’s canyon to be in Zone A: areas of the 100-year flood; base flood elevations and flood hazard factors not determined.

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Figure 5: Existing West Abutment, and Downstream Bank

6.0 RIVER ASSESSMENT

Upstream of the existing bridge crossing, the Sun River is a moderate velocity mountain river with a large diversion dam located approximately 0.86 river miles upstream. In this section the river channel makes a large sweeping turn from an eastwardly to a southwardly direction. The river channel slopes at approximately 0.74% upstream of the existing bridge. The bank full width gradually constricts as it approaches the bridge. In this section the bank full channel width ranges from about 250 feet to about 160 feet. The ordinary highwater depth is approximately 6 feet. The 50-year flood depth is approximately 14.4 feet.

Upstream of the existing bridge site, the river channel is stable and characterized by cobbles, boulders, and zones of exposed bedrock across the channel bottom with an estimated Manning’s (n) value of 0.04. The east and west banks of the river are generally stable and lined with large trees and heavy brush. The channel is not aggrading or degrading in this area. The upstream river channel is shown in Figure 6.

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Figure 6: Existing Upstream Channel

Through the existing bridge crossing, the river channel slope flattens to slope of about 0.3%. Channel width remains relatively constant with a bank full width around 160 feet and an ordinary highwater depth of approximately 7.4 feet. The 50-year flood depth is approximately 15.2 feet. Channel material remains constant with zones of cobbles, boulders, and exposed bedrock. No pier scour was noted at either the existing western or eastern bridge piers. No significant pier scour or woody debris was noted at the center pier.

Downstream of the existing bridge site, the river channel is generally consistent with that of the bridge crossing site described earlier and is straight for approximately 0.5 river miles downstream. The channel slopes at an approximate 0.3% slope. Channel width is relatively constant with a bank full width around 160 feet. The 50-year flood depth is approximately 15 feet. The channel bottom material remains similar to the upstream reach with zones of cobbles, boulders, and exposed bedrock. The main channel banks downstream of the bridge site are stable and well vegetated with large trees and heavy brush extending above ordinary highwater level of the river. The downstream channel is shown in Figure 7.

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Figure 7: Existing Downstream Channel

Downstream of the existing bridge and near the proposed crossing location a gradation of the existing streambed material was visually estimated and is summarized in Table 2.

Table 2: Existing Streambed Gradation (Visually Estimated)

River Name Gradation

D15 (ft) D50 (ft) D85 (ft) D95 (ft)

Sun River 0.054 0.386 0.71 0.0.2.19476

An overview of Sun River bridge site is shown in Figure 8. Additional photos of the river channel are included in Appendix A.

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Figure 8: Sun River Site Overview

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7.0 HYDROLOGY

7.1 DRAINAGE BASIN DELINEATION

The drainage basin associated with the Sun River at the project crossing was delineated with the USGS StreamStats program and cross referenced with available as-built information and bridge inspection reports. At the project crossing, the Sun River basin encompasses 610 square miles extending to the west into the Rocky Mountain Front. A drainage basin delineation map along with a listing of basin characteristics is included in Appendix B.

7.2 HYDROLOGIC METHODS AND ANALYSIS

There is a USGS gage, gage station number 06080900, attached to the center pier of the existing bridge.

A USGS Data Release: Peak-flow frequency analyses for selected streamgages in and near Teton County, Montana, based on data through water year 2019 contains peak flow data calculated from the gage data at this site. A discussion of this analysis is included in the following sections of this memorandum.

7.2.1 Design Flood Frequency and Freeboard Requirements

Per chapter 7.4.3.3.1 of the Federal Highway Administration Project Development and Design Manual (PDDM), the design flood for a bridge structure is the 50-year event for both Low and High Standard roadways. The check flood event is the greater of the 100-year flood or the overtopping flood as the standard check flood for water surface increase caused by the crossing.

Freeboard requirements for the Sun River bridge are set in accordance with Section 7.4.3.4.1 of the PDDM.

There is a potential for woody debris at the proposed bridge site so a design freeboard between 3.5 feet and 5.0 feet is required. Meeting the freeboard requirements will not be an issue for the proposed bridge as it is perched 78.8 feet above the ordinary high-water elevation.

7.2.2 USGS Stream Gage Data Analysis and Flow Summary

As stated earlier, the project relevant portion of the Sun River basin is located entirely within Montana’s Northwest Hydrologic Region and USGS stream gage 06080900 is located at the existing bridge. Peak flow data for the site was published in a USGS Data Release: Peak-flow frequency analyses for selected streamgages in and near Teton County, Montana, based on data through water year 2019. In this publication statistical methods are used to draw peak flow conclusions from gage data at a site as well as standard errors of prediction that define a probable range of accuracy. This peak flow data was accessed through the USGS StreamStats Program. Peak flow data is summarized in Table 3 and is also included in Appendix B.

Stream gage data was also acquired from the stream gage attached to the existing bridge. This data included a reading for the 1964 flood which overtopped the Gibson Dam. The 1964 flood flow is shown in Table 3 and the stream gage data is included in Appendix B.

Table 3: Sun River Peak Flows

River Name Q2

(cfs) Q10 (cfs)

Q25 (cfs)

Q50 (cfs)

Q100 (cfs)

Q200 (cfs)

Q500 (cfs)

1964 Flood (cfs)

Sun River 4,490 11,900 17,400 22,400 28,200 34,800 45,300 59,700

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8.0 BRIDGE ANALYSIS AND RECOMMENDATIONS

The existing Sun River Bridge provides a critical access route for Greenfields Irrigation District personnel to access both ends of an existing irrigation siphon crossing under the Sun River parallel to the existing bridge. A 75-mile detour would be required to access both ends of the siphon should the bridge be impassable. Additionally, the bridge provides the shortest route to the community of Augusta, Montana for several residences located on the east side of the river.

A discussion of the hydraulic analysis of the existing and proposed bridge structures is included in the following sections of this memorandum.

8.1 EXISTING CONDITION HEC-RAS MODEL AND CALIBRATION

The Hydrologic Engineering Center River Analysis System (HEC-RAS) Program, Version 6.4.1, was used to create a working model of the Sun River and the existing bridge structure. Site survey data was overlayed onto existing LIDAR data to create an existing surface used to generate river cross sections. River cross sections were entered into the existing conditions HEC-RAS model left to right looking downstream beginning 1,100 feet upstream of the existing Sun River Bridge and ending 1,400 feet downstream of the existing bridge. Five channel cross sections were entered upstream of the existing bridge. Ten additional cross sections were entered downstream the existing bridge.

The existing bridge information was gathered from as-built information, available survey data at the bridge site and field measurements. This data was used to enter the existing bridge structure, abutments, and pier structures within the Existing Conditions model at River Station (RS) 1400. Ineffective flow areas were then entered into the model per the recommendations in the HEC-RAS User’s Manual and following a 1:2 flow expansion downstream of the existing bridge and a 1:1 flow contraction upstream of the existing bridge.

Normal depth was used as a downstream boundary condition.

USGS Stream Gage 06080900 is located on the center pier of the existing Sun River Bridge and is shown in Figure 9. During a May 2023 field review, an observed gage reading of 4.34 ft corresponded to a water surface 34’-5.5” below the top of the existing bridge deck above the gage. The top of the bridge deck in this area was surveyed at an elevation of 4,402.04 feet. With this data, a Water Surface Elevation (WSE) was calculated at a gage height of 4.34 feet equaling 4,367.58 feet. Guage data from this site indicates that a Q2 flow rate of 4,090 cfs corresponds to a gage height reading of 8.45 feet. A Q2 WSE was then calculated at the gage site by adding the difference between the Q2 recorded gage height and the observed gage height to the observed WSE. This resulted in a Q2 WSE equaling 4,371.69 feet. The calculated Q2 WSE was used in calibrating the existing conditions model.

Approximate Ordinary High Water (OHW) elevations were surveyed at multiple locations downstream of the existing bridge. These locations were visually determined by vegetation breaks and debris lines noted during the May 2023 field review. The OHW elevations were surveyed at 4,370.37 feet and 4,371.63 feet and are assumed to correlate to the Q2 WSE. Observed OHW elevations were entered into the Existing Conditions model as observed water surface elevations and used in calibrating the Existing Conditions model.

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Figure 9: Stream Gage at Existing Bridge Center Pier

Calibration of the Existing Conditions HEC-RAS model was completed through manipulation of the channel and overbank Manning’s (n) values. Manning’s (n) values along the river channel and overbank areas were adjusted iteratively until the Q2 water surface profile generally became a best fit line through the calculated Q2 WSE at the existing bridge and observed OHW mark elevations further downstream. Manning’s (n) values were selected from Table 3-1 of the HEC-RAS Hydraulic Reference Manual and compared with site photos of the existing river and bridge. Manning’s (n) values were set to 0.04 along the channel, corresponding to Table 3-1’s lower range of a Mountain Stream with a bottom of cobbles with large boulders. Along the channel overbanks, field notes and site photos showed large trees and heavy brush.

The Manning’s (n) values across the over bank areas were set at 0.11 corresponding to Table 3-1’s mid-lower range of a floodplain covered with heavy timber with flow into the branches.

A bridge inspection of the Sun River Bridge performed in August of 2017 indicated that the low chord of the bridge has sustained damage. Inspectors believe that this damage came from the 1964 flood. Guage data recorded the 1964 flood flow at 59,700 cfs. Running this flow through the existing conditions model resulted in a 1964 Flood WSE of 4,388.88 feet which corresponds to freeboard of -0.92 feet. This indicates that the Existing Conditions model reasonably represents the 1964 flood impacting the existing bridge structure.

The 2017 bridge inspection report is included in Appendix C. Existing conditions HEC-RAS input data is displayed in Appendix D and analysis output tables are included in Appendix E.

8.2 PROPOSED BRIDGE HEC-RAS MODEL

The proposed bridge structure for this crossing is a 19’-6” wide, 455 foot long, three-span structure measured centerline of bearing to centerline of bearing. This structure spans the width of the river’s canyon opening about 270 feet downstream of the existing bridge. The proposed bridge has vertical abutments at both ends that tie into the canyon walls. The eastern abutment extends deepest into the canyon tying into

Page | 12 existing ground at an approximate elevation of 4,422 feet. The proposed bridge has two piers, both located outside the river’s bank full channel area. The bridge span from the abutments to each of the piers is 140 feet. Between the piers, the bridge spans 175 feet. This proposed bridge has an approximate structure depth of 6.5 feet. The design low chord elevations are 4,449.24 feet at the east abutment and 4,459.96 feet at the western abutment.

The proposed bridge structure was entered into a proposed condition HEC-RAS model at its planned location which corresponds to RS 1130. Ineffective flow areas were not needed since the abutments are perched well above the 500-year flow event. The existing bridge was kept in the proposed conditions HEC- RAS model since funding for the removal of the existing bridge is not guaranteed. If bridge removal funding is obtained, only the bridge superstructure will be removed. Given the close proximity of the siphon to the existing bridge, the bridge piers and abutments will remain in place to reduce the chance of damaging the siphon. The existing bridge superstructure was only impacted during the 1964 flood event so its inclusion into the proposed model will have little impact on design flow profiles.

Ice and debris were not identified as a significant issue with this crossing location and were not included in the hydraulic model. Additionally, the proposed bridge structure spans the entire bank full river channel and will have negligible impacts to the fish passage characteristics of the Sun River.

The proposed condition HEC-RAS model was evaluated for comparison with the existing conditions and results indicate that the new bridge is a hydraulic improvement in comparison with the existing. The proposed bridge provides 69.8 feet of freeboard evaluated at the Q100 design flow. The freeboard calculation is based on the controlling low chord elevation of the proposed bridge at the eastern abutment.

The low chord elevation across the channel opening is significantly higher due to the longitudinal grade of the bridge. Therefore, the actual freeboard provided by the proposed bridge is higher across the main channel.

The hydraulics of the proposed bridge are compared with the hydraulics of the existing bridge at the Q50 and Q100 flow rates in Table 4. Proposed bridge HEC-RAS input data is displayed in Appendix F. Output tables from the proposed bridge HEC-RAS analysis and a detailed hydraulic comparison of the existing and proposed bridge structures are included in Appendix G.

Table 4: Sun River Bridge Hydraulic Comparison

Structure Condition

WSE at Bridge Approach

XS

(ft)

Freeboard at Upstream Bridge Face

(ft)

Backwater Change at Bridge Approach

XS

(ft)

Channel Velocity at Bridge Approach

XS

(fps)

Q50 Q100 Q50 Q100 Q50 Q100 Q50 Q100

Existing two-span, 222’ Steel

Truss Bridge

Existing 4379.82 4381.67 8.52 6.73 10.22 11.25

Proposed 4379.82 4381.68 8.51 6.70 0.00 0.01 10.22 11.24

Proposed three-span, 455’ Conc.

Bridge

Existing 4378.01 4379.73 12.15 13.22

Proposed 4378.02 4379.77 71.46 69.79 0.01 0.04 12.13 13.19

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8.3 PROPOSED BRIDGE SCOUR

Contraction, pier, and abutment scour depths were not calculated for the proposed bridge structure.

Contraction scour was deemed negligible as the proposed bridge structure and abutments do not encroach on the river’s floodplain. Abutment scour was also not calculated. The proposed eastern abutment extends down into the river canyon the deepest to an elevation of 4,422 feet and the calculated WSE of the 1964 flood through the proposed bridge is 4,387.22 feet. As the 1964 flood was estimated to be greater than the Q500 event, it is highly probable that floodwaters will never reach an elevation required to impact the proposed bridge abutments. Pier scour was also deemed negligible and not calculated as the river channel is incised in a deep bedrock canyon. Site visits to the proposed bridge location documented visible outcrops of bedrock near the proposed bridge pier locations as shown in Figure 10. The proposed bridge piers will be securely anchored into existing bedrock mitigating potential pier scour concerns. The above scour discussion of the proposed bridge is summarized in Table 5.

Figure 10: Exposed Bedrock at Proposed Bridge Location

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Table 5: Proposed Bridge Scour

Structure Flow Frequency

Contraction Scour Depth Clear Water

(ft)

Pier Scour Depth

Course Bed (ft)

Abutment (Total) Scour Depth NCHRP

(ft) Total Scour Elevation LT

(ft)

Total Scour Elevation RT

(ft) (East) LT (West) RT

Proposed three-span, 455’ Conc.

Bridge

50-year 0.0 Anchored in

Bedrock

0.0 0.0 N/A N/A

100-year 0.0 Anchored in

Bedrock

0.0 0.0 N/A N/A

200-year 0.0 Anchored in

Bedrock

0.0 0.0 N/A N/A

8.4 PROPOSED BRIDGE ABUTMENT RIPRAP

Abutment riprap will not be needed at the proposed bridge abutments. As stated in the previous section, the proposed eastern abutment extends down into the river’s canyon the deepest to an elevation of 4,422 feet. The calculated WSE of the 1964 flood through the proposed bridge is 4,387.22 feet and is 34.78 feet below the proposed bridge abutment. It is highly probable that floodwaters will never reach an elevation required to impact the abutments. Therefore, abutment riprap is not necessary.

8.5 OTHER CONCERNS AT THE PROPOSED BRIDGE

Currently, funding is not guaranteed for the removal of the existing bridge upstream of the proposed bridge location. The existing conditions HEC-RAS model and an August 2017 Bridge Inspection Report provide evidence that the existing bridge structure was impacted during the 1964 Flood. A similar or greater flood event could compromise the existing bridge structure and subsequently pose a threat to the proposed bridge piers downstream.

APPENDIX A

Existing Siphon and Bridge Photographs

Photo 1: Existing Syphon Inlet

Photo 2: Existing Syphon Outlet

Photo 3: Existing Bridge East Abutment

Photo 4: Existing Bridge East Abutment, South Side

Photo 5: Existing Bridge East Abutment, North Side

Photo 6: Existing Bridge Center Pier, Upstream

Photo 7: Existing Bridge Center Pier, Upstream

Photo 8: Existing Bridge Center Pier, Downstream

Photo 9: Existing Bridge West Abutment, South Side

Photo 10: Existing Bridge West Abutment, South Side

Photo 11: Existing Bridge Deck, Looking East e

Photo 12: Existing Channel Bottom Downstream

Photo 13: Existing Upstream Channel Photo 14: Existing Downstream Channel

Photo 15: Existing Bridge, Looking Downstream

Photo 16: Looking Upstream from Proposed Bridge Location

Photo 17: Looking Downstream from Proposed Bridge Location

Photo 18: Looking Downstream from Proposed Bridge Location

Photo 19: Channel Bottom at Proposed Bridge Location

Photo 20: Channel Bottom at Proposed Bridge Location

Photo 21: Channel Bottom at Proposed Bridge Location

Photo 22: East Bank at Proposed Bridge Location

APPENDIX B

Sun River Hydrology Analysis

Sun River Bridge Replacement, MT FLAP BOR 2980(1) APPENDIX B - HYDOLOGIC DATA USGS 06080900 Sun River bl Diversion Dam nr Augusta MT - Peak Streamflow Records agency_cd site_no peak_dt peak_tm peak_va peak_cd gage_ht gage_ht_cd year_last_pk ag_dt ag_tm ag_gage_ht ag_gage_ht_cd 5s 15s 10d 6s 8s 33s 8s 27s 4s 10d 6s 8s 27s

USGS 06080900 1964-06-09 59700 5,7

USGS 06080900 1968-06-10 2620 5 7.59

USGS 06080900 1969-06-06 3630 5 8.04

USGS 06080900 1970-06-05 7350 5 10.01

USGS 06080900 1971-05-29 7820 5 10.78

USGS 06080900 1972-06-02 8910 2,5 11.35

USGS 06080900 1973-05-30 562 5 5.14 2 1972-12-10 7.13 1

USGS 06080900 1974-06-17 7210 5 10.08

USGS 06080900 1975-06-19 32000 5 19.00 1964

USGS 06080900 1976-05-14 8470 5 11.11

USGS 06080900 1977-05-12 536 5 4.92

USGS 06080900 1978-06-07 6330 5 9.96

USGS 06080900 1979-05-27 5380 5 9.38

USGS 06080900 1980-05-26 5430 5 9.36

USGS 06080900 2016-05-28 1110 5 5.74

USGS 06080900 2017-06-02 3840 5 8.30

USGS 06080900 2018-06-19 05:45 10500 5 11.96 2 2018-06-19 0215 11.97

USGS 06080900 2019-05-27 18:45 5070 5 9.06

USGS 06080900 2020-06-01 08:15 4490 5 8.45

USGS 06080900 2021-06-05 22:00 3000 5 7.39

# U.S. Geological Survey # National Water Information System # Retrieved: 2023-08-10 18:33:13 EDT # ---------------------------------- WARNING ---------------------------------------- # Some of the data that you have obtained from this U.S. Geological Survey database may not have received Director's approval. Any such data values are qualified as provisional and are subject to revision. Provisional data are released on the condition that neither the USGS nor the United States Government may be held liable for any # damages resulting from its use.

# More data may be available offline.

# For more information on these data, contact USGS Water Data Inquiries.

# This file contains the annual peak streamflow data.

# This information includes the following fields:

# agency_cd Agency Code # site_no USGS station number # peak_dt Date of peak streamflow (format YYYY-MM-DD) # peak_tm Time of peak streamflow (24 hour format, 00:00 - 23:59) # peak_va Annual peak streamflow value in cfs # peak_cd Peak Discharge-Qualification codes (see explanation below) # gage_ht Gage height for the associated peak streamflow in feet # gage_ht_cd Gage height qualification codes # year_last_pk Peak streamflow reported is the highest since this year # ag_dt Date of maximum gage-height for water year (if not concurrent with peak) # ag_tm Time of maximum gage-height for water year (if not concurrent with peak # ag_gage_ht maximum Gage height for water year in feet (if not concurrent with peak # ag_gage_ht_cd maximum Gage height code # Sites in this file include:

# USGS 06080900 Sun River bl Diversion Dam nr Augusta MT # Peak Streamflow-Qualification Codes(peak_cd):

# 1 ... Discharge is a Maximum Daily Average # 2 ... Discharge is an Estimate # 3 ... Discharge affected by Dam Failure # 4 ... Discharge less than indicated value, which is Minimum Recordable Discharge at this site # 5 ... Discharge affected to unknown degree by Regulation or Diversion # 6 ... Discharge affected by Regulation or Diversion # 7 ... Discharge is an Historic Peak # 8 ... Discharge actually greater than indicated value # 9 ... Discharge due to Snowmelt, Hurricane, Ice-Jam or Debris Dam breakup # A ... Year of occurrence is unknown or not exact # Bd ... Day of occurrence is unknown or not exact # Bm ... Month of occurrence is unknown or not exact # C ... All or part of the record affected by Urbanization, Mining, Agricultural changes, Channelization, or other # F ... Peak supplied by another agency # O ... Opportunistic value not from systematic data collection # R ... Revised # Gage height qualification codes(gage_ht_cd,ag_gage_ht_cd):

# 1 ... Gage height affected by backwater # 2 ... Gage height not the maximum for the year # 3 ... Gage height at different site and(or) datum # 4 ... Gage height below minimum recordable elevation # 5 ... Gage height is an estimate # 6 ... Gage datum changed during this year # 7 ... Debris, mud, or hyper-concentrated flow # 8 ... Gage height tidally affected # Bd ... Day of occurrence is unknown or not exact # Bm ... Month of occurrence is unknown or not exact # F ... Peak supplied by another agency # R ... Revised

APPENDIX C

Existing Sun River Bridge Inspection Report

FOR OFFICIAL USE ONLY

Bridge Inspection Report

Sun River Bridge

Inspection Type: Routine

PISHKUN ROAD OVER NORTH FORK OF THE SUN RIVER

Great Plains Region

Montana Area Office

NBI #: MTA-SR-001

Owned By: RECLAMATION Inspected

By:

GP-2200 Maintained

By:

Greenfield

Irrigation District

Date of Inspection: 08/30/2017-08/31/2017

Date of Previous Inspection: 03/18/2015

Time of Inspection: 8:30 AM Temperature: 80 °F

Weather: Sunny, Smoky

Latitude: 47° 37’ 06.46” Longitude: -112° 41’ 31.27”

Bridge Name: Sun River Bridge Inspection Date: 08/30/2017 – 08/31/2017

NBI #: MTA-SR-001 Area Office: Montana Area Office

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LOCATION MAP

Region: 5

State: 308 – Montana

County: 30049 - Teton

Facility Carried: Pishkun Road

Landmark Intersected: North Fork of the Sun River

General Location: 18.8 miles NW of Augusta, Montana

Cell Phone Service available: Yes

Members of the Bridge Inspection Team present on site during the Routine Bridge Inspection are as follows:

Gary Grassel (GP-2200) Great Plains Regional Office

Morgan Clapshaw (MT-432) Montana Area Office

Dan Staton (84-57000) Denver Office

Anthony Lampert (ALB-212) Albuquerque Area Office

Fracture Critical Inspection Team members:

Jeff Ticknor (GP-2400) Great Plains Regional Office

Nathan Morgan (WY-4302) Wyoming Area Office

Adam Northrup (EC-1800) Eastern Colorado Area Office

Scott Foster (MP-430) Mid-Pacific Regional Office

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Bridge Description:

The Sun River Bridge was originally built in 1916 as part of the Sun River Project to primarily support an 8-foot diameter wood stave water siphon positioned slightly above the bottom chord of the steel truss system. The original design represented the intention for lightly loaded vehicles to be able to travel across this structure primarily for maintenance purposes. At some point in time during the mid-1900’s, the wood stave siphon was removed and replaced with a concrete siphon that was relocated to pass under the river. The original driving surface comprised of 3x wood decking supported by the existing 8-inch deep steel “I” shaped stringers. Around

1982 the existing wood decking was replaced with precast concrete panels (6.5-inches deep x 3-feet long x 13-feet width). The concrete decking panels are not secured to the steel support stringers and therefore stringers are consider to be fully unbraced. Around approximately 1988, the right (south) abutment approach was removed and replaced with a concrete retaining wall. During this same time frame, the left (north) abutment approach was also modified with the addition of a concrete retaining wall and was partially backfilled with riprap.

The primary superstructure is a two-span, steel truss bridge supported on concrete abutments and a central concrete pier. The bridge roadway deck is located approximately 3-feet 6-inch below the top of truss top chord members. The deck load is transferred to the truss through stringers and floor beams. Truss panel points are interconnected to other members with rivets and gusset plates. The bridge has a fixed bearing connection at the center pier cap. Supports at the abutments are roller bearings to allow longitudinal movement for expansion and contraction of the bridge spans. The bridge spans are 110.5 feet, center-to-center of the bearings. The total structure length is approximately 249 feet 8 inches including the left abutment approach span. The clear travel width of the structure is approximately 13 feet 4 inches.

Current anticipated traffic consists of passenger and recreational vehicles. The average daily traffic is estimated at 30 vehicles. The bridge is primarily used by Reclamation, Greenfields Irrigation District, local landowners and the general public for access to the United States National Forest Service. The bridge spans the Lewis and

Clark/Teton county line.

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Bridge Plan and Elevation (From Drawing No. 28-600-677)

NOTES:

River flow is down on the page. Right Abutment is therefore on left side of page.

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Bridge Numbering Convention Looking Upstream (From Drawing No. 28-C-2)

NOTES:

Right abutment is on left side of page. This view and numbering convention is the opposite of Great Plains Regions standard, however, the convention is used here to match previous reports and conventions.

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Status of Outstanding O&M Recommendations

Definitions

Critical

Finding

A structural or safety related deficiency that requires immediate follow-up inspection or action. A Critical Finding may be designated as a Category 1 or

Category 2, as established by Reclamations D&S. Each Category will establish a required time frame to respond to such Findings.

Category 1 Recommendations involving the correction of severe deficiencies where immediate responsive action is required to ensure structural safety and operational integrity of a facility.

Category 2 Recommendations covering a wide range of important matters where action is needed to prevent or reduce further damage or preclude possible operational failure of a facility.

Category 3 Recommendations covering less important matters but believed to be sound and beneficial suggestions to improve, or enhance, the O&M of a project or facility.

Category 1

Was a Critical Finding previously identified during prior inspections? No

If yes, describe the Critical Finding:

None

Category 2

2001-2-B: Install single lane bridge, curve, speed limit, and load rating signs.

Status: Complete. See Photos 21 and 22 of this report. Per previous email correspondence with MTAO, these signs were installed sometime during the summer of 2016.

2003-2-A: Repair concrete footing (pier) on the NE corner of the north approach span.

Status: Deleted and restated as 2017-2-B.

2011-2-A: Clean gravel and other debris surrounding bearings on the south end of the Sun River

Bridge.

Status: Complete. 2015 Inspection Report indicated this Recommendation had been performed.

2012-2-A: Local emergency agencies/services need to be contacted and informed of the load ratings on the bridge.

Status: Complete. Updated on 8/18/2015. The District has been in contact with the local volunteer fire response.

2012-2-B: Eroded areas on or near the north approach span need to be backfilled.

Status: Incomplete. See Photo 6.

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2012-2-C: Replace decayed portions of the timber deck on the north approach span.

Status: Complete. 2015 Inspection Report indicated this Recommendation had been performed.

2012-2-D: Conduct a climb team inspection of the steel trusses to determine their condition and extent of damage. Upon completion, the overall condition of the bridge should be reviewed to determine its ability to carry anticipated loads and to identify repair needs especially with respect to the north approach span. A determination should be made if the bridge should be replaced.

Status: Complete. The TSC conducted a Rope Access Inspection in August 2014 documented in an April 2015 report.

2015-2-A: Attach precast decking to superstructure to resist lateral movement of the decking.

Status: Incomplete. See Photos 7 & 17.

2015-2-B: Perform visual monitoring of the deformed members between L1 and L4 of the upstream truss of Span B during routine inspections.

Status: Complete. This is not a DSIS recommendation but rather a note to future inspectors.

2015-2-C: Clear debris from the bridge bearings at both abutments.

Status: Incomplete. 2015 Bridge Inspection report indicated this recommendation was accomplished. However, this item will be listed as a new Recommendation in 2017 due to additional deterioration occurring over the previous two years. The condition in 2015 was in satisfactory condition, however, during 2017 Inspection it was apparent material had accumulated again. Therefore, a new Recommendation will be indicated in 2017.

Category 3

None.

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NEW O&M Recommendations based on 2017 GP2200 Inspection

Critical Findings

Was a Critical Finding identified during this inspection? No

If yes, describe the Critical Finding:

None

Critical Finding Plan of Action

If a Critical Finding was identified, what is the required Plan of Action to correct the issue?

None

Category 2

2017-2-A: Sun River Bridge _ At Right Abutment, Downstream and Upstream roller bearings.

1. Clean roller bearing plate of debris to prevent corrosion and to allow for unobstructed longitudinal movement. This recommendation should be performed as an annual maintenance item.

2. Jack up truss and re-position roller bearing assembly onto abutment steel bearing plate. Lubricate bearings to allow these elements to operate as intended.

3. Design and install a new truss bearing plate restraint system.

2017-2-B: Sun River Bridge _ Remove left approach boulders, located below approach span, and replace existing pier and footing. Pier is located to the far left of approach system on the downstream side of bridge.

2017-2-C: Sun River Bridge _ Install object markers (OM3 series) and 90 degree corner signs

(W1-1 series) at each end of bridge approach in accordance with latest edition of

MUTCD.

2017-2-D: Sun River Bridge _ Reconstruct left abutment approach with a concrete retaining structure similar to the right abutment approach.

2017-2-E: Sun River Bridge _ Replace all missing rivets with A325 bolts and matching washers/nuts. Several locations were identified and these locations are identified as follows: (Reference Elevation on Page 6 of this Report.)

1. Upstream Span A: A L2, A L6, A U1;

2. Upstream Span B: B L2, B L6;

3. Downstream Span A: A L4 and;

4. Downstream Span B: B L2.

2017-2-F: Sun River Bridge _ Tighten all loose nuts on bolted connections. Locations identified are on the upstream truss, in Span B: B L4, B U4 and B U7.

Category 3

2017-3-A: Drill weep holes in the right (south) abutment retaining walls.

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NOTES:

Future inspectors should monitor and review the following items to establish a stable or active condition:

1. Review all locations where Recommendations have been made.

2. Hairline crack in concrete foundation wall located at the right abutment, upstream approach face. (See Photo 15)

3. Erosion at right abutment, downstream side of approach. (See Photo 16)

4. Bent bottom chord of truss and bracing members indicated in 2017 Photos directory. (See

Photos 17 – 19)

O&M recommendations related to this bridge can be found under Sun River Bridge in DSIS.

Structural Evaluation Summary

Overall, the Sun River Bridge is in poor condition. Several items identified need to be addressed in the near future to provide for safe travel to the public and all users of this structure.

An isolated concrete pier located on the downstream side of the left abutment approach is severely deteriorated.

Riprap placed within the approach span has added a substantial lateral load to the structure and therefore could displace supporting structure off the pier, leading to failure of the approach span (Recommendation 2003-2-A deleted and restated as 2017-2-B). (See Photo 5)

Substantial loss of support material below the concrete jersey approach barriers at the left (north) end approach has been noted in the past. The area has deteriorated to the point that the barriers are spanning across eroded areas and resting against the inadequate approach guardrail posts. This area needs to be regraded to provide adequate support for the existing approach rail system (Recommendation 2012-2-B). (See Photos 6 &12)

The precast concrete decking material is not attached to the superstructure, allowing for lateral movement of the precast concrete decking. The precast decking must be attached to the superstructure to restrict lateral movement and to increase the overall load rating of the bridge (Recommendation 2015-2-A). (See Photo 7)

Currently, the roller bearings located at the right abutment, on both downstream and upstream sides, are not capable of serving the design intent. Anchor rods have failed and roller bearings have rolled off of bearing plates.

See (Recommendation 2017-2-A). (See Photos 8, 9 & 10). A new anchorage restraint assembly needs to be designed and installed to secure the primary support systems for this structure from adverse lateral movement.

Object markers and warning signs are not present, or are installed in locations that these are not visible to those travelling the road. Review the latest edition of MUTCD to address the appropriate signs required and locations of installation (Recommendation 2017-2-C). (See Photos 3, 4 & 11). Example of object marker size (OM3 series), locations (MUTCD Figure 2A-2) and height (4 feet above roadway, min.) should be in accordance with the MUTCD. An example of the ninety degree corner signs are the W1-1 series.

The performance of recommendation 2017-2-D will address several of the issues of concern that are identified at the left approach.

Replace several missing rivets. Observed locations are noted in 2017-2-E. (See Photo 13)

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Field Inspection Report

Condition Rating Codes for Items 58-62, 222, 223

N Not Applicable

Serious Condition – loss of section, deterioration, spalling, or scour have seriously affected primary structural components. Local failures are possible.

Fatigue cracks in steel or shear cracks in concrete may be present.

9 Excellent Condition

Very Good Condition – no problems noted.

Good Condition – some minor problems.

Critical Condition – advanced deterioration of primary structural elements. Fatigue cracks in steel or shear cracks in concrete may be present or scour may have removed substructure support. Unless closely monitored it may be necessary to close the bridge until corrective action is taken.

Satisfactory Condition – structural elements show some minor deterioration.

Fair Condition – all primary structural elements are sound, but may have minor section loss, cracking, spalling, or scour.

“Imminent” Failure Condition – major deterioration or section loss present in critical structural components or obvious vertical or horizontal movement affecting structure stability.

Bridge is closed to traffic, but corrective action may return structure to light service.

Poor Condition – advanced section loss, deterioration, spalling, or scour.

Failed Condition – out of service; beyond corrective action.

Condition Rating Code Equivalents for Sub-Components

G = Good (Codes 7 – 9) F = Fair (Codes 5 – 6) P = Poor (Codes 0 – 4) N = Not Applicable

58 Deck Condition 4

1. Deck Slab P Notes (with Sub-Component Number):

2. Expansion Joints N 58.1: Deck slab consists of pre-cast concrete panels bearing on unbraced steel beams. Deck slab panels are free to move laterally and need to be attached to the steel stringers. North approach span is wood decking on steel stringers. Wood decking is not attached to stringers.

58.4 & 58.9: Travel across the unsecured pre-cast panels generates a sensation of vibration and flexibility in system with light duty truck loads.

58.6: The top chord of the truss system serves as the guardrail system for this bridge. This condition is not desirable to have a compression member of the primary system subject to vehicle impact.

3. Wearing Surface G

4. Rideability F

5. Curbs, Sidewalks, and Median N

6. Parapets and/or Railing P

7. Drains and Drainage N

8. Utilities N

9. Deflection and/or Vibration P

10. Other N

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Condition Rating Code Equivalents for Sub-Components

G = Good (Codes 7 – 9) F = Fair (Codes 5 – 6) P = Poor (Codes 0 – 4) N = Not Applicable

59 Superstructure Condition 4

1. Bearing Devices P Notes (with Sub-Component Number):

2. Girders or Beams N 59.1: Right abutment U/S truss bearing plate is fully extended and exterior anchor rod is bent 30-45 degrees. Interior anchor rod head is sheared off. Right abutment D/S interior roller bearing anchor rod is sheared off. Excessive dirt and debris has impeded roller functionality.

59.4: Floor beams and stringers are unbraced over the full span length.

59.5: Overall, truss members are in satisfactory condition with minor structural deterioration due to light corrosion. Some members have sustained impact damage. Identified damage is suspected to have occurred during the 1964 flood.

58.6: Some diaphragms and bracing members are bent.

58.7: At some point in history, the truss members were painted.

Currently, the paint is essentially non-existent.

59.8: Connections, in general, are in satisfactory condition with light corrosion on most. The upstream connection of Span A, L3 has more severe corrosion. Some connections have a missing rivets.

59.9: See Note 58.4.

59.10: Sharp curves at both ends of bridge.

3. Arches or Slabs N

4. Floor beams and Stringers F

5. Trusses F

6. Diaphragms and Bracing F

7. Paint P

8. Connections or Welds F

9. Deflection and/or Vibration P

10. Alignment F

11. Other

N

Condition Rating Code Equivalents for Sub-Components

G = Good (Codes 7 – 9) F = Fair (Codes 5 – 6) P = Poor (Codes 0 – 4) N = Not Applicable

60 Substructure Condition 3

Abutments Notes (with Sub-Component Number):

1. Breastwalls or Columns F 60.3: Concrete pier under the D/S far left column at the left approach (NE corner) is deteriorated beyond any reliability for support. Large boulders/rocks, located under left approach, are bearing against column and therefore induce additional lateral pressure on base plate connection and support pier that is in very poor condition.

60.4: Minor cracking and spalling exists.

2. Piles N

3. Caps and/or Bearing Seats P

4. Backwalls and/or Bulkheads F

5. Foundations G

6. Wingwalls N

7. Weep Holes N

8. Expansion Joints N

9. Erosion and/or Scour F

10. Settlement or Deflection G

11. Slope Protection N

12. Other N

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Piers Notes (with Sub-Component Number):

13. Walls and/or Columns F 60.13: Some minor…