mt08601_HydraulicsDesignMemo_Final.pdf

Prepared for:

WESTERN FEDERAL LANDS HIGHWAY DIVISION

FINAL HYDRAULIC DESIGN

MEMORANDUM

Bozeman to Bridger Mountains Trail, MT DOT 86(1) IDIQ Contract No. DTFH70-10-D-00016

Task Order No. T-14-009

May 2016

Prepared by:

ROBERT PECCIA &

ASSOCIATES

Helena, Montana

Bozeman to Bridger Mountains Trail, MT DOT 86(1)

Hydraulic Design Memorandum – Final

Page | i

TABLE OF CONTENTS

Abbreviations/Acronyms ....................................................................... iii

1.0 Project Background

2.0 This Memorandum

3.0 Drainage Characteristics

4.0 Existing Culvert Assessment

5.0 Existing Bridge Assessment

6.0 Hydrology

6.1 Drainage Basin Delineation

6.2 Hydrologic Methods

6.2.1 Design Flood Frequency

6.2.2 USGS Rural Regression Equations

6.2.3 Bridger Creek Hydrology

7.0 Hydraulics

7.1 Culvert Size Area Correlation

7.2 Project Culvert Analysis

7.3 Energy Dissipators

7.4 Fish Passage

8.0 Bridge Analysis and Recommendations

8.1 Existing Stream and Structure

8.2 Bridger Creek Flow Data

8.2.1 Transposition of Gauged Data

8.2.2 USGS Regression Equations

8.2.3 Hydrology Summary

8.3 HEC RAS Model and Calibration

8.4 Proposed Bridge

8.5 Proposed Bridge Scour

8.6 Proposed Bridge Abutment Riprap

9.0 Floodplain Encroachments

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Tables

Table 1: Existing Culverts Table 2: Drainage Basin Summary Table 3: Culvert Size Area Correlation Table 4: Hydraulic Analysis Summary Table 5: HY8 Culvert Hydraulics Table 6: Energy Dissipators Table 7: Bridger Creek Hydrology Comparison Table 8: Bridger Creek Bridge Hydraulic Comparison Table 9: Proposed Pedestrian Bridge Scour

Figures

Figure 1: Project Vicinity Map

Appendices

Appendix A: Existing Drainage Culvert and Bridge Photographs

Appendix B: Drainage Basin Delineation Maps

Appendix C: Hydrologic Calculations and Culvert Size / Drainage Area Correlation

Appendix D: Proposed Culvert Design Analysis

Appendix E: Preliminary Hydraulic Report, BR 86-1(23)3

Appendix F: HEC RAS Model and Bridge Analysis

Appendix G: Bridge Scour Analysis and Abutment Riprap Sizing Analysis

Appendix H: FEMA Floodplain Maps

Page | iii

ABBREVIATIONS/ACRONYMS

cfs cubic feet per second

CMP Corrugated Metal Pipe

GSP Galvanized Steel Pipe

X/P Any Appropriate Material Except Plastic

FHWA Federal Highway Administration

FEMA Federal Emergency Management Agency

FETS Flared End Terminal Section fps feet per second

HEC-RAS Hydrologic Engineering Center River Analysis System

MDT Montana Department of Transportation

PDDM Project Development and Design Manual

Q# #-year design flow

RCP Reinforced Concrete Pipe

RCPA Reinforced Concrete Pipe Arch

RPA Robert Peccia and Associates

SH State Highway

USFS United States Forest Service

USGS United States Geological Survey

WRIR Water-Resources Investigation Report

WSE water surface elevation

Hydraulic Design Memorandum – Final Page | 1

1.0 PROJECT BACKGROUND

The Federal Highway Administration (FHWA) is proposing to complete the design documentation for the Bozeman to Bridger Mountains Trail project. The proposed project comprises an approximately 2.1 mile long section of pedestrian/bicycle path along Montana State Highway (SH) 86 northeast of Bozeman, MT.

The proposed path would start at the intersection of Story Mill Road and SH 86 and proceed northeasterly to the “M” parking lot. Participating agencies on the project include FHWA, the United States Forest Service (USFS), the City of Bozeman, and the Montana Department of Transportation (MDT). Robert Peccia and Associates (RPA) was selected by FHWA to assist in the development of this project. A project vicinity map is shown in Figure 1.

Figure 1: Project Vicinity Map

The proposed path will utilize existing gravel trail segments on the south side of SH 86 across the Headlands Subdivision before crossing over to the Creekwood Subdivision on the north side of SH 86.

Portions of an existing asphalt-surfaced path through the Creekwood Subdivision will be utilized. East of the Creekwood Subdivision, the new pathway will continue along the north side of SH 86 and will cross Bridger Creek via a new pedestrian bridge. Additionally, the new path will include a pedestrian underpass underneath SH 86 connecting both the “M” trailhead parking lot and the Drinking Horse Mountain trailhead parking lot.

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2.0 THIS MEMORANDUM

The purpose of this memorandum is to document the condition of the existing drainage and bridge structures along the proposed path corridor, as well as the preliminary hydraulic analysis and design recommendations for proposed culverts and the new bridge structure over Bridger Creek. This memorandum will discuss drainage characteristics, hydrologic and hydraulic assessments, and a summary of proposed culvert and bridge hydraulic recommendations. Potential floodplain encroachments and fish passage requirements are also discussed in this memorandum.

3.0 DRAINAGE CHARACTERISTICS

The project area lies in the northwest portion of Gallatin County near Bozeman, Montana. The topography of the area is characterized as foothills associated with the Bridger Mountain range and drainage characteristics for the project corridor are generally consistent with conditions seen elsewhere in southcentral Montana. The project area falls within the Upper Yellowstone-Central Mountain hydrologic region as described in the United States Geologic Survey (USGS) Water Resources Investigations Report (WRIR) 03-4308. Timbered foothills, open ridges and farm fields dominate the area, with both gently graded and steep drainage basins contributing flow to the project alignment. Bridger Creek is the primary stream crossing along the project corridor. This creek includes approximately 65 square miles of drainage area contiguous on the south and east side of the Bridger Mountains.

4.0 EXISTING CULVERT ASSESSMENT

A culvert condition assessment was completed in August of 2014. During this assessment the existing culvert locations, types, sizes, remaining service life, deficiencies, and further analysis needs were documented. The deficiencies noted during the field assessment were generally noted as minor.

The existing condition of all drainage culverts potentially impacted by the proposed path is listed in Table 1 and photos of select existing culverts are included in Appendix A. In general, the existing culverts include reinforced concrete pipe (RCP), reinforced concrete pipe arch (RCPA) and corrugated metal pipe (CMP) culverts that range in size from 12-inch to 36-inch diameter or equivalents. Many of the existing approach and subdivision culverts include flared end terminal sections (FETS).

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Table 1: Existing Culverts

Proposed Station

Existing Culvert Field Notes Photo #

Diameter (in) Material Length (ft) Slope % Condition Service Life

1500+94 24 RCP 108 Not

Calculated Fair 10 - 20 Years SH 86, Outlet Completely Buried

Left

1502+36 RT 12 CMP 36 1.44 Fair 10 - 20 Years Approach Pipe 8,9

1504+16 RT 18 CMP 40 1.35 Fair 10 - 20 Years Approach Pipe 10,11

1508+97 RT 18 RCP 40 0.85

- - Not Reviewed, Retention Pond Culvert

1512+35 RT

18 RCP 68 2.91 - - Not Reviewed, Headlands Drive

Culvert

1513+72 LT 18 RCP 62 1.08 Good 20+ Years Approach Pipe, Headlands Drive, FETS

13,14

1516+70 RT 18 RCP 56 1.00 Good 20+ Years Headlands Drive, FETS -

1524+45

30 RCPA 66 0.62

Good 20+ Years Headlands Drive, Underneath

Existing Bike Path, FETS

1527+57 30 RCPA 66 0.57 Good 20+ Years Headlands Drive, Underneath

Existing Bike Path, FETS

1528+54 RT/LT 30 RCPA 68 0.31 Good 20+ Years Approach Pipe, Powder Park Court, FETS

15,16

1533+85 RT 15 CMP 30 2.61 Fair 10 - 20 Years Approach Pipe 17,18

1534+70 RT 15 CMP 38 1.24 Fair 10 - 20 Years Approach Pipe, Plugged 19,20

1541+98 LT 24 CMP 75 Not Calculated

Good 20+ Years SH 86, Minor Damage to FETS 25,26

1545+16 24 CMP 74 Not Calculated

Fair 10 - 20 Years Approach Pipe, Creekwood Lane, Minor Damage to FETS

27,28

1556+18 RT

24 CMP 71 2.16 Good 20+ Years Approach Pipe, Boylan Road, FETS

30,31

1564+83 RT 18 CMP 55 1.15 Good 20+ Years Approach Pipe, FETS 32,33

1566+78 RT

24 CMP 90 0.84 Good 20+ Years SH 86, Very Minor Damage to

Inlet, FETS 34,35

1579+74 RT 24 CMP 87 2.19 Fair 10 - 20 Years SH 86, Minor Damage to Outlet 36,37

1588+62 30 CMP 85 6.29 Good 20+ Years SH 86, Below Drainage Pond 38,39

1588+68 LT

4 STEEL 16 - - - Not Reviewed, Pond Outlet -

1588+74 LT

18 CMP 15 - - - Not Reviewed, Pond Outlet -

1590+00 18 CMP 84 7.56 Good 20+ Years Approach Pipe, FETS 41,42

1592+70 24 CMP 118 Not Calculated

Good 20+ Years SH 86 40

1603+06 RT 36 CMP 143 7.94 Good 20+ Years SH 86 43,44

1612+78 RT 24 CMP 252 5.98 Good 20+ Years SH 86 45,46

1612+85 RT 24 CMP 68 - - - Fish Hatchery Access Road, Culvert Buried

1619+55 RT 18 CMP 70 3.68 Fair 10 - 20 Years Approach Pipe - M Parking Lot, Minor Damage to Outlet FETS

47,48

Hydraulic Design Memorandum – Final Page | 4

5.0 EXISTING BRIDGE ASSESSMENT

Montana Department of Transportation SH 86 crosses Bridger Creek at approximately project Station 1562+91. Montana Bridge P00086003+01311 facilitating this crossing was constructed in 2005 and consists of a three span continuous cast-in-place concrete slab structure. The three span lengths are

24.6 feet, 32.8 feet, 24.6 feet for a total structure length of 84.5 feet. The deck roadway width is approximately 38.7 feet and the structure is skewed 35 degrees to the left. The deck thickness is 17 inches, and it is cast integrally with the interior piers and pinned to the end abutments. With this deck thickness, the low beam of the west abutment is set at an elevation of 4,800.8 feet and is approximately

9.54 feet above the thalweg of the stream.

The existing bridge substructure consists of concrete filled steel pipe driven piling. End abutments consist of four 16 inch x ½ inch pipe piling supporting a cast-in-place concrete abutment. Interior piers consist of four 24 inch x ¾ inch pipe piling supporting the integral deck-pier caps. The piers are also skewed 35 degrees. Abutments are protected by moderately sized riprap on both 1V:2H slopes from the channel edge to the face of the abutments.

Through the bridge crossing, Bridger Creek is a moderate-velocity mountain stream with a drainage area of approximately 65.3 square miles. The stream channel is generally stable and characterized by well-graded gravels and cobbles across the channel bottom. The channel width near the bridge site is approximately 25-feet wide and shows minor backwash and trash accumulation under the structure.

There is little evidence of steam bed deposition or scour near or through the crossing area.

Photos of the existing highway bridge and stream channel are also included in Appendix A.

6.0 HYDROLOGY

6.1 DRAINAGE BASIN DELINEATION

Drainage basins were delineated for the proposed drainage and bridge improvements associated with this project using USGS contour maps, aerial photos, and project cross section files. Drainage basins were only examined for the portion of the proposed pathway east of the Creekwood Subdivision. The proposed pathway utilizes portions of the existing paths from Story Mill Road to the end of the Creekwood Subdivision and includes existing drainage structures serving the relatively level drainage areas. While a few of these existing culverts require extensions as part of this project, the hydraulic impacts to these culverts resultant from this project are generally minor and these culverts are excluded from this analysis.

West of the Creekwood Subdivision however, the proposed pathway affects the drainage characteristics and hydraulic performance of the existing culverts to a greater extent. Through this area, drainage basins were delineated for each existing and proposed culvert for analysis. A drainage basin delineation map is included in Appendix B and a summary of drainage basin areas is shown below in Table 2.

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Table 2: Drainage Basin Summary

Drainage Basin Designation

Drainage Area (Acres)

E6000

DB_1 0.7 0%

DB-2 Lyman Creek 1587.2 52%

DB-3 Bridger Creek 41127.2 63%

DB-4 0.2 0%

DB-5 7.4 0%

DB-6 2.2 0%

DB-7 5.1 0%

DB-8 5.7 0%

DB-9 1.5 0%

DB-10 37.7 0%

DB-11 76.9 0%

DB-12 0.37 0%

DB-13 0.1 0%

DB-14 19.8 0%

DB-15 1.1 0%

DB-16 5.9 0%

DB-17 391.4 55%

DB-18 1.5 0%

DB-19 1.6 0%

DB-20 102.4 8%

DB-21 0.3 0%

DB-22 0.3 0%

DB-23 0.7 0%

DB-24 3.6 0%

6.2 HYDROLOGIC METHODS

Brief discussions of the methods for estimating peak design flows used for this analysis of the drainage culverts as well as the proposed bridge structure are included below.

6.2.1 Design Flood Frequency

Per chapter 7.1.6.1 of the Federal Highway Administration Project Development and Design Manual (PDDM), there is no clear direction for determining the design flood for a bicycle and pedestrian pathway.

Therefore, the project alignment is classified as a Low-Standard Road, and the design flood frequency for culvert designs affecting only the proposed pedestrian path is the 25-year event. However, the project alignment is generally parallel to and in close proximity of Montana State Highway (SH) 86. According to the criteria presented in Chapter 7.1.6.1 of the PDDM, this highway is classified as a High-Standard Road and the design storm for drainage structures along this facility is the 50-year event.

At multiple locations along the proposed project alignment, drainage structures convey flow across both the pedestrian path and State Highway 86. At these locations the design storm for new and modified existing drainage structures directly affecting flow across the highway will be the 50-year event. The 25-year event will be used as the design storm for all other drainage structure designs along the project corridor.

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6.2.2 USGS Rural Regression Equations

The USGS Rural Regression Equations use statistical methods to draw conclusions from data at surrounding sites. They provide not only estimated peak flows, but also standard errors of prediction that define a probable range of accuracy. The Bozeman to Bridger Mountains trail project area is located in the Upper Yellowstone-Central Mountain Region of Montana, and the variables used in the USGS equations include drainage basin area and the percentage of basin area above 6,000 feet in elevation.

Peak discharges for the drainage basins associated with this project were determined by using the USGS Rural Regression Equations (Basin and Climatic Characteristics) as described in the USGS WRIR 03- 4308. Results of the USGS Rural Regression Equation calculations for each culvert drainage area are summarized Table 4 and included in Appendix C.

6.2.3 Bridger Creek Hydrology

The hydrology evaluation for the proposed Bridger Creek Pedestrian and Bicycle Bridge is described in more detail under the Bridge Analysis and Recommendation section of this memorandum. For comparison purposes the USGS Rural Regression Equations, Transposition of Gauged Data equations, and a 2001 MDT hydraulic analysis of Bridger Creek utilizing a Federal Emergency Management Agency (FEMA) hydrology analysis were evaluated in the hydrologic analysis of Bridger Creek.

7.0 HYDRAULICS

7.1 CULVERT SIZE AREA CORRELATION

In accordance with Chapter 7 of the PDDM, the minimum size of a cross drain culvert is 24 inches and the minimum size of an approach road culvert is 18 inches for a new construction project. Since the Bozeman to Bridger Mountains Trail project will involve both types of culverts, a drainage basin correlation analysis was completed to estimate the maximum drainage area associated with each of these culvert sizes. Both an 18-inch CMP and a 24-inch CMP were analyzed at the minimum slope of 0.5 percent, minimum cover of one foot, and minimum headwater to diameter ratio of 1.5, as required by the PDDM. The culvert design software program HY8-8.7.3 was then used to calculate the maximum capacity of each culvert under these minimum design conditions. Results of this analysis indicate that under minimum conditions, an 18-inch CMP and a 24-inch CMP culvert have maximum flow capacities of

7.6 cfs and 16.6 cfs, respectively.

The calculated maximum capacity for each of the culverts where then used within the USGS Rural Regression Equations to back solve for a maximum drainage area corresponding to each culvert size at both the 25-year and 50-year design storms. These drainage areas were then compared to the delineated drainage basins shown in Table 2 to determine which existing or proposed culverts have associated drainage areas that exceed these calculated drainage area limits. In general, only proposed culverts with drainage areas exceeding the areas listed in Table 3 or the proposed culverts affecting SH 86 were hydraulically evaluated and sized as part of this project. The results of this analysis are summarized in Tables 3 and 4 and supporting documentation is included in Appendix C.

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Table 3: Culvert Size Area Correlation

Culvert Size Maximum Flow

(cfs)

Q25 Maximum Area

(Acres)

Q50 Maximum Area

(Acres)

18” CMP 7.6 24.6 12.6

24” CMP 16.6 71.6 37.7

7.2 PROJECT CULVERT ANALYSIS

The proposed trail alignment will either result in extensions to existing culverts in the case where the proposed pathway is adjacent to an existing road or along an existing pathway, or result in new cross drain culverts for the situations where the proposed trail intersects natural drainage patterns upstream of existing culverts. In general, the proposed culverts are sized and located to extend or supplement existing drainage culverts in accordance with Chapter 7 of the PDDM. The proposed culverts are generally aligned with natural flow patterns and are either sized to match existing culverts or increased to meet both the conveyance needs of the applicable design storm and the applicable headwater criteria.

The analysis and design of the proposed culverts utilized the previously calculated drainage basin flows, as well as the proposed trail alignment and profile.

Hydraulic design calculations were only completed for drainage areas larger than the maximum drainage area sizes shown in Table 3, unless otherwise selected for verification on a case by case basis. All other proposed culvert actions were assumed to meet minimum design standards based on the drainage area correlation described above. Drainage basin comparisons for all drainage features between the Creekwood Subdivision and the “M” parking lot are shown in Table 4 below.

A new 12-foot by 10-foot Reinforced Concrete Box Pedestrian Underpass is designed across MT 86 at Station 1613+00. This underpass is connected to the new pedestrian path on the north side of MT 86 and is not intended to convey drainage flow. Surface flow in this area is generated by Drainage Basin 20 encompassing an area north of the new path and Drainage Basins 21 and 22 encompassing areas between MT 86 and the new path on both the west and east sides of the underpass respectively.

Surface flow generated by Drainage Basin 20 is collected and conveyed to the south across both the new pedestrian path and MT 86 via a new 30-inch culvert located at Station 1612+78. Surface flow generated by Drainage Basins 21 and 22 is collected by FLH Type 7B inlets located in drainage swales on either side of the new underpass structure at Stations 1612+78 and 1613+15 respectively. The inlet located at Station 1612+78 is connected directly to the 30-inch culvert at that location while the inlet at 1613+15 is connected to the 30-inch culvert via a new 18-inch lateral culvert.

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Table 4: Hydraulic Analysis Summary

Proposed Station

Drainage Feature Route Proposed

Size (Inches)

Material Drainage Basins

Total Area

(Acres)

E6000

Q25 (cfs)

Analyzed with HY8?

1558+25 New Bike Path Culvert Bike Path 18 X/P DB-1 0.7 0% 0.56 No

1564+83 Existing Approach Culvert Rt.

MT 86 N/A –

Maintain existing

DB-4

thru

DB-5

7.6 0% 3.20 No

1565+25 New Bike Path Culvert Bike Path 18 X/P DB-5 7.4 0% 3.14 No

1566+78 Existing MT 86 Culvert MT 86 N/A – Maintain existing

DB-6

thru

DB-8

13.0 0% 4.75 No

1570+50 New Bike Path Culvert Bike Path 18 X/P DB-7 5.1 0% 2.39 No

1576+21 New Bike Path Culvert Bike Path 18 X/P DB-8 5.7 0% 2.60 No

1579+74 Existing MT 86 Culvert MT 86 N/A – Maintain existing

DB-9

thru

DB-10

39.2 0% 10.66 No

1579+88 New Bike Path Culvert Bike Path 24 X/P DB-10 37.7 0% 10.36 No

1588+62 Existing MT 86 Culvert (Extension)

MT 86 36 GSP DB-11

thru

DB-13

77.0 0% 17.49 Yes, See HY8

1590+20 Approach Culvert Lt. MT 86 18 X/P DB-12 thru

DB-13

0.5 0% 0.41 No

1590+62 New Bike Path Culvert Bike Path 18 X/P DB-13 0.1 0% 0.14 No

1592+70 Existing MT 86 Culvert (Extension)

MT 86 24 GSP DB-14

thru

DB-15

20.9 0% 6.73 Yes, See HY8

1592+85 New Bike Path Culvert Bike Path 18 X/P DB-15 1.1 0% 0.78 No

1601+16 Approach Culvert Lt. MT 86 18 X/P DB-16 5.9 0% 2.66 No

1603+14 Existing MT 86 Culvert (Extension)

MT 86 36 GSP DB-16

thru

DB-18

398.8 55% 32.19 Yes, See HY8

1603+25 New Bike Path Culvert Bike Path 18 X/P DB-18 1.5 0% 0.97 No

1611+04 Approach Culvert Lt. MT 86 18 X/P DB-19 1.6 0% 1.02 No

1612+78 Existing MT 86 Culvert (Replacement)

MT 86 30 C DB-20

thru

DB-22

103.0 8% 15.64 Yes, See HY8

1613+15 New Bike Path Inlet with Outlet Pipe

Bike Path 18 X/P DB-22 0.3 0% 0.30 No

1619+55 Existing Approach Culvert Rt.

MT 86 N/A –

Maintain existing

DB-23

thru

DB-24

4.3 0% 2.11 No

1620+46 New Bike Path Culvert Bike Path 18 X/P DB-24 3.6 0% 1.85 No

X/P = Any appropriate material except plastic.

GSP = Galvanized Steel Pipe C= Concrete Pipe

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The hydraulic design calculations for the applicable drainage culverts were completed using HY8-8.7.3 culvert design program. The analysis incorporated the proposed drainage basin flows, local overtopping or controlling headwater elevations, and proposed culvert data to analyze the capacity of each of the proposed culverts. Both CMP and RCP culverts were analyzed at each drainage crossing, as applicable.

Plastic culverts were not evaluated for this project due to grass fire susceptibility. Riprap headwalls have not been added to the design of the proposed culverts at this time. All new path culverts are designed as square end culverts with lengths that allow for beveled end sections if necessary. All proposed culvert extensions match the material, geometry and slope of the respective existing culvert and are intended to reuse existing end sections as applicable. Results of the HY8 analyses are summarized in Table 5 and calculations are included in Appendix D. Culvert orientation and length adjustments may be necessary through final design.

Table 5: HY8 Culvert Hydraulics

Proposed Station

Drainage Feature Route Size

(Inches) Material

Q50 (cfs)

Proposed Culvert Improvement Capacity (cfs)

HW/D ≤

1.5?

1588+62 Existing Highway Culvert - Extension

SH 86 36 CSP 27.7 85.8 Yes

1592+70 Existing Highway Culvert - Extension

SH 86 24 CSP 10.9 42.7 Yes

1603+14 Existing Highway Culvert - Extension

SH 86 36 CSP 43.2 108.0 Yes

1612+78 Existing Highway Culvert - Replacement

SH 86 30 C 22.8 26.5

Yes

7.3 ENERGY DISSIPATORS

Culvert Outlet Riprap Energy Dissipators are designed at four proposed culvert locations along the project corridor. At these locations, either the proposed culvert outlet velocity, evaluated at the 10-year flow, exceeds 10 feet per second, or the proposed culvert outlet is located on a steep slope and riprap will mitigate erosion potential. The proposed energy dissipator’s size and configuration are designed per FHWA guidelines and adjusted to meet field conditions at each location. A summary of the proposed energy dissipators is included in Table 6 and design documentation is included in Appendix D.

Table 6: Energy Dissipators

Proposed Station

Drainage Feature Route Culvert

Size (Inches)

Approx.

Culvert Slope

Proposed Riprap

1576+21 New Bike Path Culvert Bike Path 18 10 Class 2

1579+88 New Bike Path Culvert Bike Path 24 12 Class 2

1592+85 New Bike Path Culvert Bike Path 18 5 Class 2

1603+25 New Bike Path Culvert Bike Path 18 5 Class 2

1612+78 New Cross Drain MT 86 30 7 Class 2

Hydraulic Design Memorandum – Final Page | 10

7.4 FISH PASSAGE

Currently there are no existing or proposed culverts identified along the project route that require fish passage design considerations. The proposed bridge over Bridger Creek is a single span structure that should not introduce any conditions impacting fish passage within the stream.

8.0 BRIDGE ANALYSIS AND RECOMMENDATIONS

The alignment of this project crosses over Bridger Creek a short distance north of an existing highway bridge constructed in 2005 under MDT project BR 86-1(23)3. A new pedestrian bridge is proposed to facilitate this crossing and a discussion of both the hydraulic analysis and recommendations for the proposed bridge are included in the following sections of this memorandum.

The hydraulic analysis completed for the existing highway bridge is documented in the Preliminary Hydraulic Report BR 86-41(23)3 and included in Appendix E for additional information.

8.1 EXISTING STREAM AND STRUCTURE

Bridger Creek crosses the project alignment near Station 1563+00. Near the crossing, the cross section of the existing stream is generally a trapezoid with a bottom width that ranges between 25 and 30 feet, and side slopes that are generally 1V:2H. The stream bed is characterized by well graded gravels and cobbles and is moderately-graded with a calculated slope of 0.0096 ft/ft over the model reach. The stream banks are heavily vegetated with tall grass brush and trees while the extended flood plain areas included open grass areas and housing developments.

The existing highway bridge, located approximately 32 feet upstream of the alignment near Station 1562+91, is a 3-span, cast-in-place, concrete slab structure constructed in 2005. The three span lengths are 24.6 feet, 32.8 feet, 24.6 feet for a total structure length of 84.5 feet and the structure is skewed 35 degrees left. At the end of each interior span, sets of four 24-inch round piers support the integral deck-pier caps and are also skewed 35 degrees left. The road deck width is approximately 38.7 feet with a thickness is 17 inches resulting in a minimum low chord elevation of 4,800.8 feet. Abutments are protected by moderately sized riprap on both 1V:2H slopes from the channel edge to the face of the abutments.

The existing highway bridge is located approximately 150 feet downstream of an existing irrigation structure across Bridger Creek and approximately 210 feet upstream of the confluence with Lyman Creek. Lyman Creek was realigned to its current location as part of the highway bridge construction in 2005. Under the existing highway bridge, a 5-foot wide concrete animal path is located along the eastern bank to facilitate animal passage. The abutment slopes associated with this path are protected with MDT Class 2 riprap. Field investigation of the existing highway bridge showed little or no evidence of abutment, pier, or contraction scour since 2005. Photos of the existing stream and highway bridge are included in Appendix A.

8.2 BRIDGER CREEK FLOW DATA

Bridger Creek is a moderately-sized mountain stream with a 65.3 square mile drainage area calculated from the crossing location. The drainage basin is characteristic of mountainous areas and includes both foothills as well as steep mountainous terrain. Upstream of the crossing the drainage basin includes open and agricultural lands in low areas followed by foothills and mountain slopes along the southern and eastern side of the Bridger Mountains. A map of the delineated drainage basin is included in Appendix B.

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Hydrologic flow calculations for the Bridger Creek drainage basin were completed by both the Transposition of Gauged Data and USGS Regional Frequency methods. Each of these methods is described in the following sections of this memorandum. Additionally, the flood flows utilized in the hydraulic design of the existing highway bridge are included for analysis.

In 2001, the Montana Department of Transportation completed a hydraulic analysis of Bridger Creek as part of bridge replacement project BR 86-1(23)3. The flows utilized in this analysis were based on flood hydrology developed by FEMA in the late 1970’s. FEMA developed flood hydrology for Bridger Creek by developing regression equations from the analysis of ten gauges on streams in or near the study area.

The Bridger Creek flows used in the 2001 MDT hydraulic analysis were adjusted by the effects of the upstream irrigation structure that diverts a portion of the stream flow to the bridge site. These adjusted flows were applied at the upstream end of the 2001 MDT model reach and are applicable for this analysis. The FEMA flows used in this analysis are included in Table 7 and are discussed in the Preliminary Hydraulic Report BR 86-1(23)3 included in Appendix E.

8.2.1 Transposition of Gauged Data

Stream gauge transfer was completed using data from USGS stream gauge 06048500, Bridger Creek near Bozeman MT. This gauge is located approximately 1.8 miles upstream of the project site and has 28 years of record.

At the gauge site, the calculated drainage area is 62.5 square miles in comparison with the 65.3 square mile drainage area at the project site. With a drainage area that is 95% of the project drainage area, the gauge data is applied directly to the bridge site and adjusted for drainage area. The USGS gauge data is adjusted using the procedures described in the USGS WRIR 03-4308 for calculating flow at ungauged sites on gauged streams.

Additionally, a similar calculation was completed immediately downstream of the project site to account for the flow contribution from Lyman Creek. Again at the gauge site, the calculated drainage area is 62.5 square miles in comparison with the 67.8 square mile drainage area at the stream confluence. With a drainage area that is 92% of the project drainage area, the gauge data is applied directly to the confluence and adjusted for drainage area. The USGS gauge data is again adjusted using the procedures described in the USGS WRIR 03-4308 for calculating flow at ungauged sites on gauged streams.

Results of the Transposition of Gauged Data analysis are presented in Table 7 and all calculations are included in Appendix C.

8.2.2 USGS Regression Equations

The USGS Rural Regression Equations for the Upper Yellowstone-Central Mountain Region of Montana utilized drainage basin area in square miles and the percentage of basin area above 6,000 feet in elevation. Peak discharges for the Bridger Creek drainage basin at the crossing as well as the Bridger Creek basin including Lyman Creek downstream were determined by using the USGS Rural Regression Equations (Basin and Climatic Characteristics) as described in the USGS WRIR 03-4308. Results of the USGS Rural Regression Equation calculations for both drainage basin areas associated with the stream crossing are summarized in Table 7 and all calculations are included in Appendix C.

8.2.3 Hydrology Summary

For the purpose of this analysis, the 1984 FEMA study flow values used in the 2001 hydraulic analysis of the existing highway bridge are in line with the results of the Transposition of Gauged Data analysis and the results of the USGS Regional Regression Analysis. For consistency with the previous analysis completed by MDT, the 1984 FEMA study flow values, evaluated below the existing irrigation structure, will be used in the analysis of both the existing highway bridge and the proposed pedestrian bridge over Bridger Creek. Similarly, the additional flow contribution provided by Lyman Creek downstream of the exiting bridge will follow the recommended flow values used the previous analysis.

Hydraulic Design Memorandum – Final Page | 12

Table 7: Bridger Creek Hydrology Comparison

Upstream of Bridge Downstream with Lyman Creek

Recurrence Interval

USGS Equations (cfs)

Gauge Transfer

(cfs)

FEMA

(cfs)

USGS Equations (cfs)

Gauge Transfer

(cfs)

FEMA

(cfs)

Q2 284 311 312 293 321 312

Q10 687 657 625 708 675 665

Q50 1,178 1,069 920 1,213 1,094 970

Q100 1,415 1,272 1,100 1,457 1,300 1,150

Q500 2,032 1,841 1,420 2,092 1,876 1,490

8.3 HEC RAS MODEL AND CALIBRATION

The Hydrologic Engineering Center River Analysis System (HEC-RAS) Program, Version 4.1.0, was used to create a working model of Bridger Creek and the existing bridge structure. Channel cross sections were located along the analyzed reach of the stream beginning immediately downstream of the irrigation structure. In total, 11 channel cross sections were entered into the existing conditions HEC-RAS model and 5 interpolated cross section were added upstream of the existing highway bridge. The modeled reach was terminated 135 feet upstream and 1,116 feet downstream of the existing highway bridge. The boundary condition for the existing conditions model was then set with a normal depth slope of 0.0211 ft/ft. This slope is an average of the energy grade slope for the downstream-most cross sections within the model reach.

The existing bridge data was gathered from the MDT BR 86-1(23)3 construction plans and entered into the model directly. Ineffective flow areas were then entered into the model per the recommendations in the HEC-RAS User’s Manual and both the bridge and bounding cross sections were skewed 35 degrees to represent the effective hydraulic opening of the existing bridge.

Calibration of the existing HEC-RAS model was accomplished by adjusting the Manning’s (n) values at each cross section corresponding to the observed vegetation breaks. 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 stream and bridge. Manning’s (n) values for the majority of main channel were set to 0.045, corresponding to the upper range for a mountain stream with no vegetation in the channel and steep banks with trees and brush. Along the channel overbanks, the site photos showed a significant change in bank vegetation that ranges from open grass areas to brush and trees. The Manning’s (n) values across the over bank area were estimated from aerial photography and range between 0.05 and 0.15.

With no known water surface elevations or high water marks available, the existing conditions model was calibrated by adjusting the channel Manning’s (n) values iteratively until the Q2, Q50 and Q100 water surface elevations nearly matched the design water surface elevations listed in the BR 86-1(23)3 construction plans at the upstream face of the existing bridge. Output tables from the existing conditions HEC RAS analysis are included in Appendix F.

Hydraulic Design Memorandum – Final Page | 13

8.4 PROPOSED BRIDGE

The proposed pedestrian bridge for this crossing includes a 14-foot wide, single-span structure that spans the width of the stream approximately 32 feet downstream of the existing highway bridge. The proposed 95-foot bridge has a clear span length of 89 feet and 1:2 abutment slopes are included at both ends. This proposed bridge has an estimated structure depth of 3.8 feet that results in a low chord elevation of

4800.4 based on the path profile.

A HEC-RAS model of a proposed bridge was evaluated for comparison with the existing conditions. The proposed bridge structure was entered into the HEC-RAS model directly and ineffective flow areas where adjusted per the recommendations in the HEC-RAS User’s Manual to account for the 1:2 flow expansions downstream of the existing highway bridge and the 1:1 flow contractions upstream of the proposed pedestrian bridge. Again, both the proposed bridge and bounding cross sections were skewed 35 degrees to represent the effective hydraulic opening of the bridge. At this skew angle, the proposed pedestrian bridge has an effective bottom width of approximately 38.5 feet.

Across the proposed bridge site, the overbank and main channel Manning’s (n) values are reduced from the values used in the exiting conditions HEC-RAS model. This reduction was completed over River Stations 1991.785 and 2046.845 to better represent the new path footprint, and abutment riprap in comparison with the heavy brush and deep grass along the existing channel. In the overbank areas, the footprint of the new pedestrian path will replace dense brush along the existing floodway with grassed slopes. In this area, overbank Manning’s (n) values are reduced from 0.15 to 0.05 representing high grass pasture. Additionally, the heavy brush along the existing channel will be replaced with abutment riprap and a new wildlife path. This change does not affect the entire channel area however and main channel Manning’s (n) values are reduced 5% to represent a slightly smoother channel. Between River Stations 1991.785 and 2046.845 the main channel Manning’s (n) values are entered at 0.042.

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 stream channel and will have negligible impacts to the fish passage characteristics of Bridger Creek.

Results of the proposed bridge model indicate that the new bridge is within the hydraulic shadow of the existing highway bridge and does not have a significant impact on the existing hydraulic characteristics of Bridger Creek. The new bridge provides 3.75 feet of freeboard evaluated at the Q100 flow and slightly raises the water surface elevations through the crossing area compared to the existing bridge. However, both upstream of the existing highway bridge and downstream of the proposed pedestrian bridge, Q100 water surface elevations generally match the existing condition water surface elevation.

The hydraulics of the proposed pedestrian bridge are compared with the hydraulics of the existing highway bridge at the Q50 and Q100 flow rates in Table 8 and output tables from the HEC-RAS analysis are included in Appendix F.

Table 8: Bridger Creek Bridge Hydraulic Comparison

Structure WSE at Upstream Section

RS 2047

(ft)

WSE at Upstream Section

RS 2128

(ft)

Backwater

RS 2128

(ft)

Avg. Velocity Downstream of Structure RS 1992

(fps)

Q50 Q100 Q50 Q100 Q50 Q100 Q50 Q100

Existing 3-span, 84.5- Concrete

Bridge

4,796.30 4,796.71 4,796.99 4,797.43 2.99 3.18

Proposed single-span, 95-foot Ped.

Bridge

4,796.32 4,796.76 4,796.98 4,797.43 -0.01 0.00 3.00 3.16

Hydraulic Design Memorandum – Final Page | 14

The HEC-RAS analysis indicated a proposed pedestrian bridge spanning the existing channel and designed with a minimum low chord elevation of 4800.4 feet is hydraulically similar to the existing highway bridge at all evaluated flows.

8.5 PROPOSED BRIDGE SCOUR

Contraction scour, associated with proposed bridge does not appear applicable as the proposed abutment slopes associated with the new bridge are a continuation of the existing highway bridge abutments. Stream flow is contracted by the existing highway bridge and remains contracted through the proposed pedestrian bridge. Therefore, no contraction scour calculations are completed for the proposed pedestrian bridge. Similarly, pier scour associated with the proposed bridge is not applicable as the proposed pedestrian bridge is a single span structure and no piers are included in the proposed bridge layout.

Abutment scour was calculated for the 50-year, 100-year and 200-year flows for the proposed pedestrian bridge opening. Abutment scour was calculated based on spill-through abutments and both the distance the abutment protrudes into the overbank flow and the discharge cut off by the abutment are calculated at the 50-year, 100-year and 200-year flows. While the 200-year flow data is not available from the FEMA flow data previously discussed, this value was interpolated between the known 100-year and 500-year data. The resultant abutment scour calculations show moderate scour on both abutments.

While the proposed pedestrian bridge is within the hydraulic shadow of the existing highway bridge, the calculated abutment scour depths remain a concern and should be mitigated with abutment riprap.

Abutment riprap size is discussed in the following section of this report. All scour calculations are included in Appendix G, and results of the scour analysis are summarized in Table 9. Scour elevations are based on a channel bottom elevation of 4,791.23 feet.

Table 9: Proposed Pedestrian Bridge Scour

Structure Flow Frequency

Abutment Scour Depth (ft)

Abutment Scour Elevation (ft)

Left Left Right Right

Proposed single-span, 95-foot Ped.

Bridge

50-year 1.3 2.3 4,789.93 4,788.93

100-year 2.0 3.0 4,789.23 4,788.23

200-year 2.6 3.6 4,788.63 4,787.63

8.6 PROPOSED BRIDGE ABUTMENT RIPRAP

Riprap sizing calculations for the proposed pedestrian bridge abutments followed the guidelines and equations presented in Design Guideline 14.3 of HEC 23, Vol II. Results of the analysis indicate that Class 1 Riprap will adequately protect the proposed bridge abutments up to the 200-year event flow.

However, the existing highway bridge abutments are protected with MDT Class 2 riprap which is equivalent to FHWA Class 4 riprap. To maintain continuity along the stream banks, FHWA Class 4 riprap is recommended for the proposed pedestrian bridge abutments. Results of the riprap sizing calculations are included in Appendix G.

Hydraulic Design Memorandum – Final Page | 15

9.0 FLOODPLAIN ENCROACHMENTS

FEMA-delineated floodplains for Bridger Creek cross the proposed pathway at several locations. Between Story Mill Road and the Bridger Creek bridge, the pathway will occasionally be adjacent to or within a “Zone X” floodplain, coinciding with potential flooding from 100-year to 500-year floods. Refer to Appendix H for FEMA floodplain maps, or “firmettes”, of the project area for more detail. Zone X floodplains are one of the lower risk floodplain zones that do not require flood insurance, but may see flooding periodically. Fill slope stabilization measures, including riprap or vegetative measures, may be appropriate along the pathway at specific locations and will be addressed in later design phases.

Up to 200 feet on either side of the proposed Bridger Creek pedestrian and bicycle bridge will be within a “Zone AE” flood zone. Zone AE flood zones refer to locations associated with the 100-year flood where base flood elevations and flood hazard factors have been modeled and determined. The addition of the Bridger Creek pathway bridge represents a potential for base flood elevation impacts. However, HEC- RAS modeling indicates that the proposed bridge does not raise the 100-year flood elevation of Bridge Creek upstream of the existing highway bridge.

APPENDIX A

Existing Drainage Culvert and Bridge Photographs

Photo 6: 1500+94, SH 86 Culvert, 24” RCP Photo 8: 1502+36 RT, Approach Culvert, 12” CMP

Photo 9: 1502+36 RT, Approach Culvert, 12” CMP Photo 10: 1504+16 RT, Approach Culvert, 18” CMP

Photo 11: 1504+16 RT, Approach Culvert, 18” CMP Photo 13: 1513+72 RT, Headlands Drive, 18” RCP

Photo 14: 1513+72 RT, Headlands Drive, 18” RCP Photo 15: 1528+54 RT, Headlands Drive, 30” RCPA

Photo 16: 1528+54 RT, Headlands Drive, 30” RCPA Photo 1: 1533+85 RT, Approach Culvert, 15” CMP

Photo 18: 1533+85 RT, Approach Culvert, 15” CMP Photo 19: 1534+70 RT, Approach Culvert, 15” CMP

Photo 20: 1534+70 RT, Approach Culvert, 15” CMP Photo 25: 1541+98 LT, SH 86 Culvert, 24” CMP

Photo 2: 1541+98 LT, SH 86 Culvert, 24” CMP Photo 3: 1545+16, Approach Culvert, 24” CMP

Photo 28: 1545+16, Approach Culvert, 24” CMP Photo 30: 1556+18 RT, Approach Culvert, 24” CMP

Photo 31: 1556+18 RT, Approach Culvert, 24” CMP Photo 32: 1564+83 RT, Approach Culvert, 18” CMP

Photo 33: 1564+83 RT, Approach Culvert, 18” CMP Photo 34: 1566+78 RT, SH 86 Culvert, 24” CMP

Photo 35: 1566+78 RT, SH 86 Culvert, 24” CMP Photo 36:1579+74 RT, SH 86 Culvert, 24” CMP

Photo 37: 1579+74 RT, SH 86 Culvert, 24” CMP Photo 4: 1588+62, SH 86 Culvert, 30” CMP

Photo 5: 1588+62, SH 86 Culvert, 30” CMP Photo 40: 1592+70, SH 86 Culvert, 24” CMP

Photo 41: 1590+00, Approach Culvert, 18” CMP Photo 42: 1590+00, Approach Culvert, 18” CMP

Photo 43: 1603+06 RT, SH 86 Culvert, 36” CMP Photo 44: 1603+06 RT, SH Culvert, 36” CMP

Photo 45: 1612+78 RT, SH 86 Culvert, 24” CMP Photo 46: 1612+78 RT, SH 86 Culvert, 24” CMP

Photo 47: 1619+55 RT, Approach Culvert, 18” CMP Photo 48: 1619+55 RT, Approach Culvert, 18” CMP

Photo 1: SH 86 Bridge, 1562+91 Looking Northeast

Photo 2: SH 86 Bridge, 1562+91 Looking Southeast

Photo 3: SH 86 Bridge, 1562+91 Looking Southwest

Photo 4: SH 86 Bridge, 1562+91 Downstream Side Looking East, Pier 2 in Foreground

Photo 5: SH 86 Bridge, 1562+91 Substructure Looking East, Pier 2 in Foreground

Photo 6: SH 86 Bridge, 1562+91 Looking Downstream at Bridger Creek

APPENDIX B

Drainage Basin Delineation Maps

Se rv ic e

La ye r C re di ts

: C op yr ig ht

N at io na l G eo gr ap hi c So ci et y, i-cu be d C on te nt m ay n ot re fle ct

N at io na l G eo gr ap hi c' s cu rre nt m ap p ol ic y.

So ur ce s:

DB

-15

DB

-17

DB

-24

DB

-23

DB

-20

DB

-22

DB

-19

DB

-18

DB

-16

DB

-14

DB

-13

DB

-9

DB

-11

DB

-10

DB

-6

DB

-8

DB

-7

DB

-4

DB

-5

DB

-2

Lym an

Cre ek

DB

-3

Bri dg er Cre ek

DB

-1

DB

-12

DB

-21

89 8989

P ro je ct

L o ca tio n

D at e:

2/

/2

P at h:

F :\h ig hw ay s\

FH

W

A\

_B oz em an _T ra il\

G

IS

\D ra in ag eB as in s\

D ra in ag e_ Ba si ns

_l et te r_

R

EV

IS

ED

.m xd

Bo zem an Tr ail Dra ina ge

Ba sin s Ga llat in

Co un ty, MT

1:

,6

Ma p L eg en d D ra in a g e B a si n B o u n d a ry

DB

Pa ge o f 3

P ro je ct

A lig n m e n t

Fe et ic e

La ye r C re di ts

: C op yr ig ht

N at io na l G eo gr ap hi c So ci et y, i-cu be d C on te nt m ay n ot re fle ct

N at io na l G eo gr ap hi c' s cu rre nt m ap p ol ic y.

So ur ce s:

DB

-15

DB

-17

DB

-23

DB

-20

DB

-19

DB

-14

DB

-11

DB

-10

DB

-5

DB

-2

Lym an

Cre ek

DB

-3

Bri dg er Cre ek

DB

-1

DB

-12

DB

-4

DB

-6

DB

-7DB

-8 DB

-9 DB

-13

DB

-16

DB

-18

DB

-21DB

-22

DB

-24 86

89 8989

P ro je ct

L o ca tio n

D at e:

2/

/2

P at h:

F :\h ig hw ay s\

FH

W

A\

_B oz em an _T ra il\

G

IS

\D ra in ag eB as in s\

D ra in ag e_ Ba si ns

_l et te r_

R

EV

IS

ED

.m xd

Bo zem an Tr ail Dra ina ge

Ba sin s Ga llat in

Co un ty, MT

Ma p L eg en d D ra in a g e B a si n B o u n d a ry

DB

Pa ge o f 3

1, 3, Fe et

1:

6, ,0

-f o o t

C o n to u r

In te rv a l

P ro je ct

A lig n m e n t ic e

La ye r C re di ts

: C op yr ig ht

N at io na l G eo gr ap hi c So ci et y, i-cu be d C on te nt m ay n ot re fle ct

N at io na l G eo gr ap hi c' s cu rre nt m ap p ol ic y.

So ur ce s:

Bri dg er Cre ek

Bri dg er Cre ek < 6

Bri dg er Cre ek

89 8989

P ro je ct

L o ca tio n

4, 9, Fe et

D at e:

2/

/2

P at h:

F :\h ig hw ay s\

FH

W

A\

_B oz em an _T ra il\

G

IS

\D ra in ag eB as in s\

D ra in ag e_ Ba si ns

_L ay ou t_ le tte r.m xd

Bo zem an Tr ail Dra ina ge

Ba sin s Ga llat in

Co un ty, MT

1:

8, Ma p L eg en d D ra in a g e B a si n B o u n d a ry

DB

Pa ge o f 3

,0

-f o o t

C o n to u r

In te rv a l

P ro je ct

A lig n m e n t

APPENDIX C

Hydrologic Calculations

Culvert Size / Drainage Area Correlation

Bozeman To Bridger Mountains Trail Maximum 18" CSP Drainage Area for HW/D = 1.5, 0.5% Slope 24.6 acres MT DOT 86(1) Maximum 24" CSP Drainage Area for HW/D = 1.5, 0.5% Slope 71.6 acres

1/14/2016

Bike Path Station

Drainage Feature Route Existing

Size Existing Material

Existing Invert In.

Existing Invert Out

Slope Length Proposed

Size Proposed Material

Proposed Invert In.

Proposed Invert Out

Slope Length Headwater Elevation

Headwater Station

Headwater Location

Indiviulal Drainage Basins

Combined Drainage Basins Indiviual

Area (acre) Combined Area (acre)

E6000 Q10 (cfs) Q25 (cfs) Q50 (cfs) Analyzed in HY8?

1508+87 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4741.09 4739.82 3.74 34 1512+21 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4742.88 4742.08 2.22 36 1524+45 New Bike Path Culvert (Extension) Bike Path 18 RCPA 4758.43 4758.02 0.62 66.23 18 Equiv. C 6 1527+57 New Bike Path Culvert (Extension) Bike Path 18 RCPA 4761.9 4761.52 0.58 66.05 18 Equiv. C 6 1541+50 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4779.07 4778.37 2.5 28 1541+96 New Bike Path Culvert (Extension) Bike Path 24 CSP 4777.19 4776.25 0.97 96.6 24 GSP 20 1558+25 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4793.67 4793.07 2.00 30 DB_1 DB-1 0.7 0.7 0% 0.24 0.56 No, Acceptable Size 1562+91 Bridger Creek Bridge N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A DB-2 Lyman Creek DB-2 Lyman Creek 1587.2 1587.2 52% 58.55 89.31 Used in HEC RAS Model 1562+91 Bridger Creek Bridge Bike Path N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A DB-3 Bridger Creek DB-3 Bridger Creek thru DB_ 24 41127.2 41793.0 63% 686.73 955.00 Used in HEC RAS Model 1564+83 Existing Approach Culvert Rt. MT 86 18 CSP 4800.09 4799.46 1.15 54.82 DB-4 DB-4 thru DB-5 0.2 7.6 0% 1.47 3.20 No, Acceptable Size 1565+25 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4801.54 4800.52 3.64 28 DB-5 DB-5 7.4 7.4 0% 1.44 3.14 No, Acceptable Size 1570+50 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4815.51 4813.20 4.81 48 4817.51 1569+25 Ditch Lt. DB-7 DB-7 5.1 5.1 0% 1.09 2.39 No, Acceptable Size 1576+21 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4853.50 4849.52 9.95 40 4854.11 1576+00 Ditch Lt. DB-8 DB-8 5.7 5.7 0% 1.18 2.60 No, Acceptable Size 1579+74 Existing MT 86 Culvert MT 86 24 CSP 4840.54 4838.63 2.19 87.4 DB_9 DB_9 thru DB_10 1.5 39.2 0% 5.09 10.66 No, Acceptable Size 1579+88 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 24 X/P 4854.11 4849.64 6.21 72 4860.37 1579+25 Ditch Lt. DB-10 DB-10 37.7 37.7 0% 4.94 10.36 No, Acceptable Size 1588+62 Existing MT 86 Culvert (Extension) MT 86 36 CSP 4862.34 4857.01 6.29 84.7 36 GSP 4863.35 4862.34 6.28 16 4873.22 1587+50 Path DB-11 DB-11 thru DB-13 76.9 77.0 0% 8.50 17.49 27.70 Yes, See HY8 1590+20 Approach Culvert Lt. MT 86 18 CSP 4881.90 4875.53 7.56 84.3 18 X/P 4881.50 4879.02 5.64 44 DB-12 DB-12 thru DB-13 0.37 0.5 0% 0.18 0.41 No, Acceptable Size 1590+62 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4882.91 4882.75 0.50 32 DB-13 DB-13 0.1 0.1 0% 0.06 0.14 No, Acceptable Size 1592+70 Existing MT 86 Culvert (Extension) MT 86 24 CSP 4874.53 4866.78 7.95 97.478 24 GSP 4876.60 4874.53 7.95 26 4889.23 1592+25 Ditch Lt. DB-14 DB-14 thru DB-15 19.8 20.9 0% 3.16 6.73 10.90 Yes, See HY8 1592+85 New Bike Path Culvert Bike Path N/A N/A N/A N/A N/A N/A 18 X/P 4887.81 4886.01 5.00 36 4889.10 1592+25 Ditch Rt. DB-15 DB-15 1.1 1.1 0% 0.34 0.78 No, Acceptable Size 1601+16 Approach Culvert Lt.