Beaverhead River, Ruby River and Splits Enhanced Hydraulic Analysis Report Madison County, Mt

Home , Beaverhead River, Gravelly Range, Ruby Range (Montana), Ruby River

January 20 20

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS ENHANCED HYDRAULIC ANALYSIS AND FLOODPLAIN MAPPING REPORT MADISON COUNTY, MT

PROJECT #18-154

Beaverhead River, Ruby River and Splits Enhanced Hydraulic Analysis and Floodplain Mapping Report Madison County, Montana

Contract No.: WO-AESI-184 Mapping Activity Statement No.: 2018-01

January 17, 2020

Prepared for: Montana Department of Natural Resources and Conservation 1424 9th Avenue Helena, MT 59620-1601 (406) 444-6816

Prepared by: Allied Engineering Services, Inc. 32 Discovery Drive Bozeman, MT 59718 (406) 582-0221

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

TABLE OF CONTENTS TABLE OF CONTENTS ...... I FIGURES ...... II TABLES ...... II APPENDICES ...... III 1 INTRODUCTION & BACKGROUND ...... 1 1.1 Community Description ...... 3 1.1.1 General Overview ...... 3 1.1.2 Historical Flooding ...... 4 1.2 Basin Descriptions ...... 4 1.3 Previous Studies ...... 6 2 HYDROLOGIC ANALYSIS ...... 6 3 HYDRAULIC ANALYSIS ...... 7 3.1 Methodology and Hydraulic Model Setup ...... 7 3.2 Field Survey and Topographic Information ...... 10 3.2.1 LiDAR Collection ...... 10 3.2.2 Field Survey Collection ...... 10 3.3 Profile Baseline ...... 11 3.4 Boundary Conditions ...... 12 3.5 Manning’s Roughness Coefficients ...... 13 3.6 Development of Cross-Sectional Geometries ...... 14 3.7 Hydraulic Structures ...... 17 3.8 Non-Conveyance/Blocked Obstruction Areas ...... 17 3.9 Letter of Map Revision and Existing Study Data Incorporation ...... 18 3.10 Split Flow Analysis ...... 18 3.11 Multiple/Worst Case Scenario Analysis ...... 22 3.12 Model Calibration ...... 24 3.12.1 Historical Sources ...... 24 3.12.2 Gage Data ...... 24 3.13 Floodway Analysis ...... 25 3.14 cHECk-RAS ...... 26 3.15 Other Special Hydraulic Modeling Considerations ...... 26

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

4 FLOODPLAIN MAPPING ...... 27 4.1 Floodplain Work Maps ...... 27 4.2 Effective Tie-In Locations ...... 28 4.3 Changes Since Last FIRM Mapping – 1-Percent-Annual-Chance Flood Event Comparison ..... 28 4.4 Changes Since Last FIRM Mapping – Floodway Comparison ...... 28 4.5 Floodplain Boundary Standard Audit ...... 28 4.6 Flood Depth Grids ...... 28 4.7 Water Surface Elevation Grids ...... 29 5 FLOOD INSURANCE STUDY ...... 29 5.1 FIS Text ...... 29 5.2 Floodway Data Tables ...... 29 5.3 Water Surface Elevation Profiles ...... 29 6 REFERENCES ...... 29

FIGURES Figure 1-1. Primary Flooding Sources ...... 2 Figure 3-1. Photo of the Ruby River Lower (left photo) and Upper (right photo) Reach...... 14 Figure 4-2. Cross Sections for the Twin Bridges Highway 41 Bridge Superimposed on 2D Model Results...... 15

TABLES Table 1-1. Flooding Sources Studied ...... 1 Table 1-2. Description of Hydraulic Analysis Options ...... 3 Table 1-3. Census Population Estimates ...... 4 Table 1-4. Census Housing Units Estimates ...... 4 Table 2-1. Discharges Recommended from Hydrologic Analyses ...... 7 Table 3-1. Station Ranges for Hydraulic Model Reaches ...... 8 Table 3-2. Model Plans ...... 9 Table 3-3. Field Survey Collection Summary ...... 11 Table 3-4. Summary of Station References ...... 12 Table 3-5. Boundary Conditions ...... 13 Table 3-6. Manning’s n Values used in Hydraulic Model ...... 14

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-7. Channel Modification Data ...... 16 Table 3-8. Split Flow Descriptions ...... 19 Table 3-9. Lateral Weir Coefficients ...... 19 Table 3-10. Optimization Sequence for the Beaverhead River - Twin Bridges Reach and the Ruby River ...... 21 Table 4-11. Locations of Worst Case Scenario Analyses...... 23 Table 3-12. USGS Stream Gage Data Used in Model Calibration ...... 24 Table 3-13. Shallow Flooding Sources ...... 26

APPENDICES Appendix A – Certificate of Compliance Appendix B – Hydraulic Work Maps Appendix C – Effective FIRM Maps Appendix D – Watershed Work Maps Appendix E – Flow Diagram Maps Appendix F – Study Area Photographs Appendix G – Modeled Cross Section Geometries at Structures Appendix H – Hydraulic Analysis Tables Appendix I – cHECk-RAS Appendix J – Changes Since Last FIRM – 1-Percent-Annual-Chance Flood Event Comparison Appendix K – Change Since Last FIRM – Floodway Comparison Appendix L – LOMC Location Maps Appendix M – Flood Insurance Study - Text Appendix N – Flood Insurance Study – Floodway Data Tables Appendix O – Flood Insurance Study - Profiles

iii

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

1 INTRODUCTION & BACKGROUND Allied Engineering Services, Inc. (AESI) completed detailed hydraulic analyses of the Beaverhead River, Ruby River, and associated split flows in Madison County. Work was completed under a contract with the Montana Department of Natural Resources and Conservation (DNRC) associated with Mapping Activity Statement 2018-01 (Madison-Jefferson Watershed, Phase II – Modernization). This report documents the hydraulic analyses and provides data for ensuing floodplain mapping efforts. Results of the analyses will be incorporated into the Madison County, Montana, and Incorporated Areas Digital Flood Insurance Rate Map (DFIRM) and Flood Insurance Study (FIS). Appendix A includes the Certification of Compliance form that confirms the study was completed using sound and accepted engineering practices and complies with all contract documents.

Other than a single Flood Insurance Rate Map (FIRM) panel in Twin Bridges (1) reflecting approximate Zone A mapping, these flooding sources have not been studied by FEMA. The purpose of this study is to replace the current Zone A mapping in Twin Bridges with Zone AE and Floodway mapping and to develop new Zone AE areas along the Beaverhead River and Ruby River in Madison County. Table 1-1 provides a list of primary flooding sources included in this hydraulic study, and Figure 1-1 shows the location of the flooding sources. Except for a 1.5-mile reach of the Beaverhead River in Twin Bridges, the studies are classified as Enhanced without Floodway and utilize Hydraulic Analysis Option D as described in Table 1-2. The Twin Bridges reach of the Beaverhead River is classified as Enhanced with Floodway and utilizes Hydraulic Analysis Option E as described in Table 1-2.

Table 1-1. Flooding Sources Studied Flooding Source Study Type Upstream Limit Downstream Primary Reach Limit Length (miles) Enhanced Boundary of Upstream Beaverhead without Beaverhead Corporate 26.3 River and Splits Floodway County and Limits of Twin (Option D) Madison County Bridges Downstream Beaverhead Enhanced with Upstream Corporate River and Splits Floodway Corporate Limits 1.5 Limits of Twin (Twin Bridges) (Option E) of Twin Bridges Bridges Enhanced Downstream Confluence Beaverhead without Corporate Limits with Big Hole 1.1 River and Splits Floodway of Twin Bridges River (Option D) Approximately Enhanced Confluence 1,230 feet Ruby River and without with downstream of 54.5 Splits Floodway Beaverhead Ruby River (Option D) River Reservoir

ALLIED ENGINEERING SERVICES, INC. 1

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Figure 1-1. Primary Flooding Sources

ALLIED ENGINEERING SERVICES, INC. 2

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 1-2. Description of Hydraulic Analysis Options Option Cross Flow Paths Manning’s n Values Structures Flood Sections (Left, Right, Zone and Channel) D Each section Reach lengths Overbanks from Included; structure A or reviewed by adjusted LULC data, channel data from as-builts, AE engineers based on draft value estimated design plans, floodplain separately and “measured” in the calibrated where field, or other possible community datasets with opening information E Each section Reach lengths Overbanks from Included; structure AE reviewed by adjusted LULC data, channel data from as-builts, engineers, based on draft value estimated design plans, channel floodplain separately and “measured” in the bathymetry calibrated where field, or other included in possible community datasets sections with opening information

The hydraulic analysis was completed using peak discharges for the 10-, 4-, 2-, 1-, and 0.2- percent-annual-chance (10-, 25-, 50-, 100-, and 500-year) flood events. The hydraulic analysis also includes the 1-percent-plus-annual-chance flood event. The hydraulic work maps in Appendix B include the floodplain mapping for the 1- and 0.2-percent-annual-chance flood events along with the floodway mapping for the Twin Bridges reach.

The hydraulic analyses and floodplain mapping completed for this project relies upon data provided by several contractors. Morrison-Maierle, Inc. completed the field surveying tasks (2) including bathymetric cross-section survey data and hydraulic structure data while Quantum Spatial provided the topographic LiDAR data (3). Michael Baker International (Baker) completed the hydrologic analysis (4). Information related to the data provided by these contractors is included in the appropriate sections of this report. 1.1 Community Description 1.1.1 General Overview The flooding sources studied are in the western half of Madison County. The Beaverhead River flows through the Town of Twin Bridges, the only incorporated town directly affected by these flooding sources. The Town of Sheridan is located approximately 2.7 miles northeast of the Ruby River.

Madison County has experienced a moderate population increase in the past 18 years while Twin Bridges has experienced a decrease. Table 1-3 summarizes the Census population data (5). Table 1-4 shows the Census housing unit estimates. With the continued development near these flooding sources, a study of the unmapped portions of these rivers is needed. This study will help the community understand the risks of living and working near these flooding sources.

ALLIED ENGINEERING SERVICES, INC. 3

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 1-3. Census Population Estimates Community 2000 2010 % Change 2017 % Change Population Population from 2000 to Population from 2010 to 2010 Estimate 2019 Twin Bridges 400 375 -6.3% 307 -18.1% Madison County 6,851 7,691 12.3% 7,902 2.7%

Table 1-4. Census Housing Units Estimates Community 2000 Housing 2010 % Change 2017 % Change Units Housing from 2000 Housing from 2010 to Units to 2010 Units 2016 Twin Bridges 216 206 -4.6% 185 -10.2% Madison County 4,671 6,940 48.6% 6,999 0.9%

1.1.2 Historical Flooding Most severe flooding events along the reaches of the Beaverhead River and the Ruby River included in this study have been the result of spring snowmelt or ice jams. Historically, notable flooding along these reaches has occurred numerous times. As recently as 2011, ice jam flooding occurred along the Beaverhead River, and flooding during the spring of 1984 near Twin Bridges and along the Ruby River was estimated to be greater than the 1-percent-annual-chance-flood event (100-year event).

Construction of the Ruby River Reservoir was completed in 1938. The reservoir is located directly upstream of the Ruby River study reach. Storage at full pool is 37,642 acre-feet covering 970 acres. As outlined in the Baker hydrology report, the USGS gage “Ruby River below reservoir near Alder” has 55 years of annual peak flow data from 1963 to 2017, after construction of the dam. The largest flood peaks occurred in 1984, 1995, 2011, 1970, and 1964. The 1984 event was nearly a 0.2-percent-annual-chance-flood event (nearly a 500-year event) and the other events ranged from roughly 10- to 4-percent-annual-chance-flood events. 1.2 Basin Descriptions The basin description text shown below for the Beaverhead River and the Ruby River was taken directly from the Baker hydrology report (4).

The Jefferson River watershed drains a substantial portion of southwest Montana, and, along with the Madison and Gallatin Rivers, is one of the three headwater tributaries that forms the Missouri River near Three Forks, MT. The Jefferson River forms at the confluence of the Beaverhead and Big Hole Rivers near Twin Bridges, MT, approximately 60 miles upstream of Three Forks. The tributaries to the Jefferson River drain the continental divide to the west (Big Hole River) and south (Beaverhead River), as well as portions of the Elkhorn Mountains (Boulder River) and the Ruby Range, Gravelly Range, and Tobacco Root Mountains (Ruby River). The Jefferson River watershed at USGS gaging station near Three Forks, MT (USGS 06036650) drains approximately 9,560 mi2.

ALLIED ENGINEERING SERVICES, INC. 4

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

From its source near Twin Bridges, the Jefferson River is a relatively low gradient, meandering river anastomosed with multiple split flows around well vegetated, quasi- permanent islands. The Jefferson River contains broad floodplains, which are inundated during relatively high flows that overtop the streambanks and continue as shallow overland flow. The floodplains have strong connectivity with the Jefferson River through the shallow ground water table present during the spring and early summer peak flows. The major tributaries to the Jefferson River (Big Hole, Beaverhead, and Ruby Rivers) share similar characteristics with the Jefferson River (low gradient, meandering channel, broad floodplains). Only the headwater streams and creeks which feed these tributaries have steep, higher gradient channels characteristic of headwater streams.

Much of the land use adjacent to the Jefferson River and floodplain is classified as agricultural (farming and ranching). While several small farming communities are present along the Jefferson River, the setting is almost entirely rural, with Three Forks having the highest population (approximately 2,000 (US Census Bureau 2016 projected)) followed by Whitehall (approximately 1,100), Twin Bridges (approximately 400), Willow Creek (approximately 200), and Cardwell (approximately 40). The largest community within the Jefferson River watershed is Dillon, MT (along the Beaverhead River) with a population of just under 4,300. US Highway 287, State Highway 55, State Highway 41, and Interstate 90 are the major roadways present along portions of the Jefferson River. These roadways, as well as numerous county roads, city streets, private drives, farm/ranch accesses, and the Montana Rail Link railroad have bridges that cross the Jefferson River.

Several small irrigation systems divert water from the Jefferson River, but these appear to be relatively minor diversions and generally deliver water to farms and ranches within, or very near, the Jefferson River floodplain. There are no impoundments on the Jefferson River, but two major impoundments are located within the watershed: Clark Canyon Dam and Reservoir on the Beaverhead River, and the Ruby Dam and Reservoir on the Ruby River. Clark Canyon Dam was completed in 1964, and the reservoir stores approximately 257,000 acre-ft. The Ruby Dam was completed in 1938, and the capacity of Ruby Reservoir is about 37,600 acre-ft. As noted above, much of the land along the Jefferson River and its tributaries is in private ownership; primarily as farms, ranches, and the businesses and residents of the communities along the rivers. Throughout the remainder of the watershed, however, most of the land ownership is public land - managed primarily by the US Forest Service, Bureau of Land Management, and State of Montana.

The Jefferson River watershed elevation ranges from 4,077 feet above MSL (NGVD29) at USGS gaging station 06036650 (Jefferson River near Three Forks, MT), to over 11,000 feet in the watershed’s mountain peaks. The mean basin elevation is 6,750 feet, and 75% of the basin is at an elevation above 6,000 ft. Approximately 33% of the watershed is forested. Annual precipitation varies widely across the watershed, with up to 50 inches per year in the high mountains and as low as 12 inches per year at the Jefferson River valley floor. Based on data collected using USGS StreamStats (McCarthy et al.

ALLIED ENGINEERING SERVICES, INC. 5

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

2016), mean annual precipitation averaged across the watershed is 19.6 inches per year. Temperatures vary widely across the watershed as well, with wintertime low temperatures frequently dropping well below zero degrees Fahrenheit, and summertime high temperatures average more than 80°F in the watershed’s lower elevations (Montana Climate Office). 1.3 Previous Studies Roughly 1.7 miles of the Beaverhead River within the corporate limits of the Town of Twin Bridges was studied in the effective Federal Emergency Management (FEMA) FIS. The effective FIRM panel 0001B has an effective date of July 3, 1986. This FIRM panel is included in Appendix C. The approximate locations where changes will be made to the Special Flood Hazard Areas (SFHA) are highlighted on the FIRM. Two SFHA zone designations are shown on the panel, Zone A and Zone C. The Zone A designation is described on the panel as “Areas of 100-year flood; base flood elevations and flood hazard factors not determined.” The Zone C designation is described on the panel as “Areas of minimal flooding. (No shading).”

2 Hydrologic Analysis Baker completed the hydrologic analyses for the Beaverhead River and Ruby River in July of 2018. Discharges for the 10-, 4-, 2-, 1-, 0.2, and 1-plus-percent-annual-chance flood events were estimated for use in the hydraulic analysis (4). The report provided a recommendation for the annual exceedance probability discharges to use in the hydraulic analysis. Select maps from the hydrology report are included in Appendix D. A summary of discharges from the hydrology report is provided in Table 2-1.

ALLIED ENGINEERING SERVICES, INC. 6

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 2-1. Discharges Recommended from Hydrologic Analyses Flooding Source and Location Peak Discharges (cfs) 10% 4% 2% 1% 0.2% 1% + Beaverhead River at Twin Bridges, 2,350 3,190 3,910 4,720 7,030 6,760 MT (USGS Gage Station 06023100) Beaverhead River above Confluence 1,344 1,670 1,924 2,177 2,795 2,907 with Ruby River (Node 600) Beaverhead River near Twin Bridges, 1,300 1,620 1,870 2,120 2,730 2,830 MT (USGS Gage Station 06018500) Ruby River near Twin Bridges, MT 1,590 2,200 2,720 3,300 4,920 6,020 (USGS Gage Station 06023000) Ruby River below Ramshorn Creek 1,320 1,880 2,360 2,900 4,450 5,310 near Sheridan, MT (USGS Gage Station 06022000) Ruby River at Laurin, MT (USGS Gage 1,080 1,610 2,100 2,670 4,400 5,940 Station 06021500) Ruby River below Ruby Reservoir 1,470 1,820 2,100 2,390 3,130 3,210 near Alder, MT (USGS Gage Station 06020600) Ruby River above Ruby Reservoir 1,580 1,990 2,330 2,700 3,730 3,800 near Alder, MT (USGS Gage Station 06019500)

While these flows provided the basis for hydraulic analysis, the occurrence of split flows resulted in different flow values at several of the flow change locations. Appendix H titled “Cross Section Discharge and Elevation Table” provides a summary of the flow changes for each mapped flooding source as they were determined and applied in the hydraulic model. For more information on split flows and how they impact peak discharges, refer to Section 3.10.

3 Hydraulic Analysis 3.1 Methodology and Hydraulic Model Setup HEC-RAS version 5.0.7 (6) was used to perform hydraulic modeling. Geometric data for the model was developed using RAS Mapper, HEC-RAS, ArcGIS (7) and GeoHECRAS (8). Multiple terrain dataset tiles were imported into HEC-RAS’s RAS Mapper and merged to create a single terrain file for modeling. Hydraulic structures were modeled in accordance with HEC-RAS User’s Manual, version 5.0 (9). Standards listed in FEMA Policy Standards for Flood Risk Analysis and Mapping (10) were also followed to ensure the study meets agency standards.

The Beaverhead River model begins upstream of its confluence with the Big Hole River and extends to the Madison/Beaverhead county boundary. The Ruby River model begins immediately above the Ruby’s confluence with the Beaverhead River and ends shortly before the Ruby River Reservoir. Table 3-1 provides starting and ending stations for the modeled reaches.

ALLIED ENGINEERING SERVICES, INC. 7

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-1. Station Ranges for Hydraulic Model Reaches Reach/Segment Begin Station End Station Beaverhead River – Twin Bridges 0 13976 Beaverhead River - Upper 13976 152508 Ruby River 0 287539

Three separate models were created for the project – two for the Beaverhead River and one for the Ruby River. The Beaverhead River was split into two models to separate the Enhanced with Floodway (Option E) study through Twin Bridges from the Enhanced without Floodway (Option D) study upstream.

Regulatory plans were created for each model. The regulatory plans apply the highest discharges from the split flow analysis using flow change locations within the steady flow file. The regulatory plans were used for determination of water surface elevations for the 10-, 4-, 2-, 1-, and 0.2-percent-annual-chance events, as well as the 1-percent-plus simulation.

The floodway plan for the Twin Bridges reach of the Beaverhead River simulates the 1-percent- annual-chance events with applied encroachments causing no more than 0.5-feet of rise in water surface elevations compared to base water surface elevations. Detailed information on floodway modeling can be found in Section 3.13 of this report. Split flow plans model the worst-case scenarios for either the primary or secondary flooding sources and include a network of lateral weirs to compute split flow quantities. See Table 3-2 for a list of the plans used in regulatory and split flow models.

ALLIED ENGINEERING SERVICES, INC. 8

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-2. Model Plans Model Plan Description Beaverhead River - Worst-Case Models regulatory flows with berms and Upper Beaverhead Upper roads intact. Lateral structures and junctions are optimized to determine split flows. Beaverhead River - Worst-Case LS 37144 Models regulatory flows with lateral Upper structure 37144 blown out. Lateral structures and junctions are optimized to determine split flows. Beaverhead River - Beaverhead Upper Models the worst-case flow rates from the Upper Regulatory split flow models. Flows are hardwired in. Beaverhead River - Twin Bridges Models regulatory flows and the Twin Twin Bridges Regulatory - Levee Bridges non-levee feature as intact by assuming flow behind the non-levee feature is ineffective. Flows are hardwired in. Beaverhead River - Twin Bridges Models regulatory flows without the Twin Twin Bridges Regulatory - No Bridges non-levee feature by assuming flow Levee behind the levee is effective. Flows are hardwired in. Beaverhead River - Twin Bridges Splits - Models regulatory flows with non-levee Twin Bridges Levee feature intact. The single lateral structure is optimized to determine split flows. Beaverhead River - Twin Bridges Splits - Models regulatory flows without the non- Twin Bridges No Levee levee feature. Lateral structures are optimized to determine split flows. Beaverhead River - Twin Bridges - Models the 1%-annual-chance event with Twin Bridges Floodway applied encroachment stations resulting in no more than 0.5 feet of rise compared to the 1%-annual-chance event without encroachment stations. The Twin Bridges Regulatory – Levee model was used as the basis for the floodway analysis. Ruby River Worst-Case Ruby Models regulatory flows with berms and roads intact and headgates closed. Lateral structures and junctions were optimized to determine split flows. Ruby River Worst-Case Splits Models regulatory flows with selected berms and roads blown out and headgates open. Lateral structures and junctions were optimized to determine split flows. Ruby River Ruby Regulatory Models the worst-case flow rates from the split flow models. Flows are hardwired in.

ALLIED ENGINEERING SERVICES, INC. 9

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

3.2 Field Survey and Topographic Information The following subsections provide a description of the topographic data used for the hydraulic analysis. Field survey and LiDAR information was collected by other contractors using the methods and procedures outlined in FEMA’s Guidelines and Specifications for Flood Risk Analysis and Mapping (Data Capture Technical Reference (11), Guidance for Flood Risk Analysis and Mapping Data Capture – General (12), and Guidance for Flood Risk Analysis and Mapping Data Capture – Workflow Details (13)). 3.2.1 LiDAR Collection Terrain data was collected October 24 - 27, 2017, for the entire study footprint in the form of LiDAR points by Quantum Spatial (3). They provided the following deliverables:

• LAS v 1.4 Points o All Classified Returns o Unclassified Flightline Swaths • 3.0 Foot ESRI Grids, Comma Delimited ASCII (*.asc), and ESRI Geodatabase o Hydroflattened Bare Earth Model (DEM) o Ground Density (ESRI Grids) • Shapefiles (*.shp) o Area of Interest o LiDAR Tile Index o Contours (1.0 foot) o Ground Survey Data o Total Area Flown o Water’s Edge Breaklines with Z Values (used in hydroflattening) o 3D Building Footprint Polygons • ESRI Geodatabase (*.gdb) o Contours (1.0 foot) Note that hydroflattening was performed for rivers that are nominally wider than 100 feet which excludes the Beaverhead River and the Ruby River. 3.2.2 Field Survey Collection Ground survey was collected for select riverine cross sections and the hydraulic structures on the Beaverhead River and the Ruby River in October and November of 2017 by Morrison-Maierle (2). A supplementary inventory was collected for Clear Creek in October and November of 2018 by Morrison-Maierle. Survey data was collected using GNSS RTK methods of survey. Trimble R8 Model-3 GNSS receivers were used, with Trimble TSC3 survey controllers and Trimble Access software. Table 3-3 lists the number of cross-section and structure surveys that were completed for each main study reach.

ALLIED ENGINEERING SERVICES, INC. 10

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-3. Field Survey Collection Summary Flooding Source and Reach Number of Number of Cross Hydraulic Sections Structures Beaverhead River – Twin Bridges 1 11 Beaverhead River - Upper 5 5 Ruby River 29 12 Ruby River – Clear Creek Split 6 0

The field survey data was presented in Montana Coordinate System, North American Datum of 1983 (NAD83-2011). Units are reported in International Feet. Elevations are referenced to the North American Vertical Datum of 1988 (NAVD88). Units are reported in U.S. Feet. GNSS-derived orthometric heights (elevations) were computed using Geoid 12B.

The Twin Bridges reach of the Beaverhead River is an Enhanced Level Option E with Floodway study reach and required a high density of surveyed cross sections to describe the channel’s bathymetry. A structure field survey was also collected for the Twin Bridges reach and includes one bridge. A sketch of the bridge structure was also provided. As the hydraulic data was being developed and after the completion of survey data collection, it was observed that a pedestrian crossing under Highway 41 was not surveyed. Since the crossing allows a portion of the flow to bypass the bridge opening, the tunnel’s dimensions were estimated from aerial photography and included in the model.

For the Enhanced Level Option D without Floodway study reaches, bathymetric survey data was collected on the Beaverhead and Ruby Rivers at a much lower density. Structures were not surveyed but had the following data recorded: type, location, structure dimensions, material type, and backwater potential. The limited detail structure inventory includes data summary tables and structure sketches.

In addition to the above referenced data, photographs of each hydraulic structure were taken to assist with the creation of the hydraulic model bridge geometries. These photographs are included in Appendix F of this report. 3.3 Profile Baseline The water line developed during the hydrologic analysis approximates the channel centerline and was used to establish the profile baselines. The water line was reviewed against the 2017 NAIP aerial photograph and the LiDAR terrain. Based on the review, minor adjustments were made to the water lines before using the linework as the final profile baseline. River stationing for cross sections and other notable features references the stream distance as measured by the profile baseline and increases from downstream to upstream. Each modeled stream and its associated station reference are shown in Table 3-4.

ALLIED ENGINEERING SERVICES, INC. 11

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-4. Summary of Station References Flooding Source Station Reference Beaverhead River Feet above confluence with Big Hole River Ruby River Feet above confluence with Beaverhead River California Slough Feet above convergence with Beaverhead River Owsley Slough Feet above convergence with Beaverhead River Split 1 Feet above convergence with Beaverhead River Split 2 Feet above convergence with Beaverhead River Split 3 Feet above convergence with Split 2 Split 4 Feet above convergence with Beaverhead River Split 5 Feet above convergence with Owsley Slough Spring Creek Feet above convergence with California Slough MainStSplit Feet above limit of study North Split Feet above limit of study Clear Creek Split Feet above convergence with Ruby River Clear Split 1 Feet above convergence with Clear Creek Jacobs Slough Feet above limit of study Jacobs Split 1 Feet above limit of study Mill Creek Feet above convergence with Ruby River Mill Split 1 Feet above convergence with Ruby River Mill Sub Split 1 Feet above convergence with Mill Sub Split 2 Mill Sub Split 2 Feet above convergence with Mill Split 1 Ruby Split 1 Feet above convergence with Clear Split 1 Ruby Split 2 Feet above limit of study

3.4 Boundary Conditions Normal depth water surface elevations or junctions with other flooding sources were used as downstream boundary conditions for the reaches. The slope of the water surface taken from the LiDAR data near the most downstream cross section was used for normal depth calculations. Table 3-5 summarizes the boundary conditions used in the analysis.

ALLIED ENGINEERING SERVICES, INC. 12

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-5. Boundary Conditions Flooding Source Boundary Condition Beaverhead River Normal Depth (0.00118 slope) Ruby River Normal Depth (0.00130 slope) California Slough Junction with Beaverhead River Owsley Slough Junction with Beaverhead River Split 1 Junction with Beaverhead River Split 2 Junction with Beaverhead River Split 3 Junction with Split 2 Split 4 Junction with Beaverhead River Split 5 Junction with Owsley Slough Spring Creek Junction with California Slough MainStSplit Normal Depth (0.00166 Slope) North Split Normal Depth (0.00130 slope) Clear Creek Split Junction with Ruby River Clear Split 1 Junction with Clear Creek Jacobs Slough Normal Depth (with 0.00230 slope) Jacobs Split 1 Normal Depth (0.00220 slope) Mill Creek Junction with Ruby River Mill Split 1 Junction with Ruby River Mill Sub Split 1 Junction with Mill Sub Split 2 Mill Sub Split 2 Junction with Mill Split 1 Ruby Split 1 Junction with Clear Split 1 Ruby Split 2 Normal Depth (0.00120 slope)

3.5 Manning’s Roughness Coefficients Manning’s roughness coefficients (Manning’s n values) were based on the 2017 NAIP aerial imagery, photographs provided by the Morrison-Maierle survey (2), and calibration at gages (see Section 3.12.2). Manning’s n values were obtained by referencing tables provided in “Open- Channel Hydraulics” (17).

The Beaverhead River channel area upstream of Twin Bridges was set to a value of 0.032. This is indicative of a clean, winding channel with some weeds and stones. Through Twin Bridges, Manning’s n was lowered in the channel to 0.028 based on calibration efforts and its character better described as clean and less sinuous. The Ruby River channel was set to a value of 0.032 except for the most upstream reach which was given a value of 0.038. Photos of the lower Ruby reaches show a clean river lacking a thick riparian corridor. Photos of the upper reach show thicker side vegetation and weedier channels (Figure 3-1). Manning’s n values for overbank areas accounted for various land uses and were applied to cross sections by drawing land use polygons estimated from aerial imagery. Figure 3-1. Photo of the Ruby River Lower (left photo) and Upper (right photo) Reach.

ALLIED ENGINEERING SERVICES, INC. 13

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-6 provides a summary of the range of Manning’s n values used.

Figure 3-1. Photo of the Ruby River Lower (left photo) and Upper (right photo) Reach.

Table 3-6. Manning’s n Values used in Hydraulic Model Land Use and Description Range of Manning’s n Values Channel 0.028-0.04 Cultivated 0.03 Dense Brush and Trees 0.1 Light Brush 0.05 Light Commercial and Light Residential 0.06 Medium Brush 0.06 Natural Field 0.04 Road 0.016 Short Grass 0.03

3.6 Development of Cross-Sectional Geometries Cross section locations were set using guidance provided in the HEC-RAS Hydraulic Reference Manual (9) as well as established floodplain modeling practice. Additionally, the 2017 LiDAR (3), a rough 2D model, and the 2018 field survey by Morrison-Maierle (2) assisted with cross section placement. The 2D model created for the Twin Bridges reach was especially helpful for cross section placement at the Highway 41 bridge. Figure 4-2 provides the cross sections layout near the bridge structure superimposed on the velocity output of the 2D model for the 100-year flood event.

ALLIED ENGINEERING SERVICES, INC. 14

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Figure 4-2. Cross Sections for the Twin Bridges Highway 41 Bridge Superimposed on 2D Model Results.

Average cross section spacing is generally less than 500 feet for all modeled reaches. In reach sections with excessive meandering, a larger spacing was used to avoid crossing the river profile more than once and prevent the overestimation of conveyance.

All cross-sectional geometries sampled the 2017 LiDAR. The Beaverhead River above Twin Bridges and the Ruby River applied LiDAR data for the entire cross section since bathymetric survey data was limited in these reaches. The Twin Bridges reach of the Beaverhead had a higher density of surveyed cross-sections given its Enhanced Level Option E with Floodway classification, and LiDAR was only used to describe overbank areas. Channel geometry for the Twin Bridges reach was described by superimposing survey data onto coincident cross sections or sampling from an interpolated surface created from the surveyed data.

Preliminary, “at-a-station” hydraulic analysis for select surveyed bathymetric cross-section locations on the Beaverhead River Upper Reach and the Ruby River indicated that the addition of the bathymetric cross-section data could lower the 1-percent-annual-chance water surface elevation roughly 0.5 feet compared to only sampling the LiDAR. As a result, a typified channel based on the surveyed bathymetric data was added to the primary flooding sources classified as Enhanced Level Option D without Floodway. A trapezoidal low-flow channel was included for the cross sections using the HEC-RAS Channel Design/Modification tool in the geometry editor (6). This tool allows the user to create a typical template that can be applied to selected cross sections.

ALLIED ENGINEERING SERVICES, INC. 15

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Templates were developed by approximating depth and flow area at each surveyed cross section. The depth at each surveyed cross section was approximated as the difference between the lowest surveyed channel elevation and the LiDAR water surface elevation. The Beaverhead River and the Ruby River were too small for hydro-flattening grade lines that would have smoothed the water surface, and a water surface elevation was approximated. The flow area was calculated as the area below the approximate LiDAR water surface elevation.

Using average calculated flow areas and depths, trapezoidal templates were produced to approximate the lost conveyance in the LiDAR data. The templates were modified slightly after running the recorded flow on the day LiDAR was flown and comparing simulated water surface elevations to the measured LiDAR water surface elevation. See Table 3-7 for information on trapezoid template dimensions and where they were applied.

Table 3-7. Channel Modification Data River General Start End Bottom Template Flow Area Location Station Station Width (FT) Depth (FT) (SQ. FT) Beaverhead Upstream of 152508 26839 43.5 2.3 105.3 River Confluence with Ruby River Beaverhead Downstream of 26007 13976 50.9 3.4 179.0 River Confluence with Ruby River Beaverhead Downstream of 5807 617 39.7 4.7 198.0 River Twin Bridges Corporate Limits Ruby River Upstream of 191 243522 24 1.8 47.3 Confluence with Beaverhead River Ruby River Upstream of 243715 287539 14 1.8 29 Clear Creek Split

For cross sections on the secondary flooding sources, cross-sectional geometries were determined using the LiDAR terrain data only. Except for Clear Creek, these flooding sources typically did not contain flowing water when the LiDAR was collected.

As recommended in the HEC-RAS Hydraulic Reference Manual (14), contraction and expansion coefficients were set as 0.1 and 0.3 in areas of gradual transition, and 0.3 and 0.5 at typical bridge sections.

Bank stations were placed at the boundary between the stream channel and the overbank/floodplain area. When possible, bank stations were placed at a topographic inflection point which provides a clear break between the stream and the overbank/floodplain. As a general criterion for choosing bank stations, grades steeper than 30% were categorized as part

ALLIED ENGINEERING SERVICES, INC. 16

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

of the channel. Several cross sections have bank stations higher than the largest events modeled because of steeply eroded banks.

Cross section numbering in the model is based on the HEC-RAS river stations. Cross section geometries at structures can be viewed in Appendix G. Photographs of cross sections adjacent to hydraulic structures are provided in Appendix F. 3.7 Hydraulic Structures Hydraulic structures were modeled in HEC-RAS using conventional engineering practice and guidance provided in the HEC-RAS Hydraulic Reference Manual (14). A total of 41 structures were surveyed and 36 were included in the hydraulic models. An irrigation pipe surveyed on the Ruby River (RBR_230) was not included in the model given its minimal obstruction to flow and questionable permanence during large events. A few headgates were also not included since they lead to isolated canals that have limited conveyance with no flooding potential (see surveyed structures BVR_050, RBR_250, RBR_270, RBR_290). The “Summary of Modeled Hydraulic Structures” table in Appendix H provides a summary of the modeled hydraulic structures.

For the only hydraulic structure on the Beaverhead River in Twin Bridges, the structure’s dimensions were configured from the collected survey data and checked against the field sketch. For all other hydraulic structures located in Enhanced Level Option D without Floodway reaches, the structure geometries were estimated from field sketches and LiDAR data only. Photographs were used to visually check the geometry of each individual hydraulic structure.

Guidance provided in the HEC-RAS Hydraulic Reference Manual was used to determine low flow and high flow hydraulic structure modeling approaches.

Appendix F provides photographs of the hydraulic structures. Hydraulic structure cross section geometries can be viewed in Appendix G. 3.8 Non-Conveyance/Blocked Obstruction Areas Ineffective areas and blocked obstructions were applied at cross sections to accurately depict areas conveying flood flows. Ineffective flow areas were used in the models for the following hydraulic scenarios:

1. Ineffective flow areas are used in the cross sections adjacent to hydraulic structures to represent the physical obstruction of the structure and represent the expansion or contraction flow path either to or from the structure. Hydraulic structure modeling guidance provided in the HEC-RAS Hydraulic Reference Manual (14) was used to place these ineffective flow areas. 2. Ineffective areas were added to the models in areas assessed to be hydraulically disconnected since flow would not be conveyed downstream in these areas. 3. Areas of backwater were modeled as ineffective flow. 4. Areas where the flow would not be in the primary direction of flow were modeled as ineffective flow areas. An example would be where an old meander comes into the river laterally at a cross section.

ALLIED ENGINEERING SERVICES, INC. 17

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

5. Areas near buildings (or in the hydraulic “shadow” of buildings) were occasionally modeled as ineffective areas. This was done to account for areas of flow that would not be active behind buildings. 6. Areas that cross lakes, ponds, and all other localized depressions.

Blocked obstructions were also used in the model as follows:

1. Blocked obstructions were placed to represent buildings or other physical obstructions in a cross section.

Ineffective flow areas and blocked obstructions were placed in accordance with engineering judgment and guidance from the HEC-RAS Hydraulic Reference Manual. A summary of cross sections with ineffective areas or blocked obstruction, along with reason for the placement of ineffective or blocked areas, is contained in the table titled “Explanation of Ineffective and Blocked Flows” in Appendix H. 3.9 Letter of Map Revision and Existing Study Data Incorporation No Letter of Map Revisions or any other existing studies were included in this analysis. 3.10 Split Flow Analysis The Beaverhead and Ruby River systems form a complex network of split flows because of the limited hydraulic capacity of the channels and adjacent floodplains. The wide river valleys contain numerous abandoned channel reaches and sloughs that convey flow during large events. These split flows form secondary flooding sources. Split flows with a significant amount of flow and depth (average depth greater than 0.5 feet) were modeled. A separate HEC-RAS split flow model was used to calculate the magnitude of the split flows. Table 3-8 lists each of these split flows, the flooding source, the split type, and the length of the split flow reach. In total, there are over 71 miles of secondary flooding sources that were modeled. Split flow names matched official geographic names where possible, and generic split names were applied elsewhere.

ALLIED ENGINEERING SERVICES, INC. 18

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-8. Split Flow Descriptions Split Flow Name Flooding Source Split Type (LS, Junction, Gate) Stream Length (miles) California Slough Beaverhead River LS 126292 16.2 Owsley Slough Beaverhead River LS 37144 2.6 Split 1 Beaverhead River LS 129608 5.4 Split 2 Beaverhead River LS 86080 2.6 Split 3 Beaverhead River LS 82552 0.7 Split 4 Beaverhead River LS 58304 1.5 Split 5 Beaverhead River LS 23940 0.4 Spring Creek Beaverhead River LS 69462 5.0 MainStSplit Beaverhead River LS 12270 1.5 North Split Beaverhead River LS 6000 0.3 Clear Creek Split Ruby River Junction CC_RR_1 11.0 Clear Split 1 Clear Creek LS 26230 1.3 Jacobs Slough Ruby River LS 27600 5.7 Jacobs Split 1 Jacobs Slough LS 11100 2.7 Mill Creek Ruby River LS 94600 5.6 Mill Split 1 Mill Creek LS 4850 3.8 Mill Sub Split 1 Mill Split 1 LS 12100 0.7 Mill Sub Split 2 Mill Split 1 LS 11750 2.1 Ruby Split 1 Ruby River LS 212800 1.1 Ruby Split 2 Ruby River LS 11350 1.2

Lateral weirs were used to calculate most split flows. Lateral weir coefficients were selected based on review of the guidance for values recommended in the document “HEC-RAS 5.0 2D Modeling User’s Manual” (15). Table 3-9 provides the range of values for the broad crested weir coefficients based on the height of the weir.

Table 3-9. Lateral Weir Coefficients Description Weir Coefficient Range Levee/Roadway – 3 ft or higher above natural ground 1.5 – 2.6 Levee/Roadway – 1-3 ft or higher above natural ground 1.0 – 2.0 Natural high ground barrier – 1-3 ft high 0.5 - 1.0 Non-Elevated overbank terrain. Lat structure not elevated 0.2 – 0.5 above ground.

Within the split flow model, junctions and lateral weirs were optimized to determine split flow quantities. The Beaverhead River - Upper split flow model plans were run several times with each iteration using the final answer from the previous run as the starting values for the lateral weirs’ initial splits. After several plan runs, the lateral split flows appeared to stabilize despite the HEC-RAS warning that “Flow Optimization Failed to Converge” for select profiles, a common warning for models with multiple lateral weirs. Flows were carefully inspected to ensure they are reasonable and maintain continuity within the system.

ALLIED ENGINEERING SERVICES, INC. 19

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

The Beaverhead River – Twin Bridges model’s plan to compute split flows assuming the non- levee feature holds (plan title: Twin Bridges Splits – Levee) only had a single lateral structure, the pedestrian underpass, and optimized in a single run. The plan to compute split flows assuming the non-levee feature fails (plan title: Twin Bridges Splits – No Levee) required separate plans to optimize the lateral structures. Flows were sequentially “hardwired in” from upstream to downstream. Additionally, the pedestrian underpass was conservatively ignored because of difficulties optimizing the high density of lateral structures near the Highway 41 bridge.

The Ruby River also required flows to be sequentially hardwired in, and only a few split flows could be optimized at a time. All splits reached convergence for the 1-, 0.2-, and 1-percent-plus- annual-chance return flow events. However, some of the flow calculation model runs failed to converge for the lower events modeled. In these cases, the simulation was examined to ensure the results were reasonable and near convergence. Table 3-10 provides the optimizing schema used to determine split flow quantities.

A flow diagram illustrating split flow quantities has been provided in Appendix E. The “Cross Section Discharge and Elevation Table” in Appendix H provides the modeled flooding discharges at each cross section. The split flows result in flow rates that vary from the original hydrologic analysis flow change locations.

ALLIED ENGINEERING SERVICES, INC. 20

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Table 3-10. Optimization Sequence for the Beaverhead River - Twin Bridges Reach and the Ruby River Split Flow Optimizing Sequence Notes Model Twin 1. Beaverhead LS 12270 and LS 10000 Bridges 2. Beaverhead LS 6000 and MainStSplit Splits – LS 6780 No Levee Worst- 1. Junction CC_RR_1 (Clear Creek/Ruby This split affects all flow quantities Case Ruby River) and Ruby LS 242100 in Clear Creek, Clear Split 1, Ruby Split 1, and Ruby River Reach 2 2. Ruby LS 228700 and Clear Creek LS 48480 3. Ruby LS 212800 and Clear Creek LS 26230 4. Ruby LS 94600 This split affects flow quantities in Mill Creek, Mill Split 1, Mill Sub Split 1, Mill Sub Split 2, And Ruby River Reaches 3 and 4 5. Mill Creek LS 8750, LS 4850, and Ruby LS 60000 6. Mill Creek Split 1 LS 12100, 11750 7. Ruby LS 27600, LS 25700, LS 25450, LS 11350, LS 9400, LS 6000, LS 4800 8. Jacobs Slough LS 11100 Worst- 1. Ruby LS 243545 and LS 242800 This split affects all flow quantities Case in Clear Creek, Clear Split 1, Ruby Splits Split 1, and Ruby River Reach 2 2. Ruby LS 228700 and Clear Creek LS 48480 3. Ruby LS 213200, 212800 and Clear Creek LS 26230 4. Ruby LS 188300 5. Ruby LS 94600 This split affects flow quantities in Mill Creek, Mill Split 1, Mill Sub Split 1, Mill Sub Split 2, And Ruby River Reaches 3 and 4 6. Mill Creek LS 8750 7. Mill Creek LS 4850, and Ruby LS 60000 8. Mill Creek Split 1 LS 12100, LS 11750 9. Ruby LS 27600, LS 25700, LS 25450, 10. Jacobs Slough LS 11100 11. Ruby LS 11350, LS 9400, LS 6000, LS Applied the Worst-Case Ruby flow 4800 rate upstream of these lateral structures since this represents the worst-case for Ruby Split 2

ALLIED ENGINEERING SERVICES, INC. 21

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

3.11 Multiple/Worst Case Scenario Analysis The Beaverhead and Ruby River valleys contain several non-levee features (often irrigation berms or roads) separating split flow reaches. Non-levee features are structures that cannot be accredited in accordance with the Code of Federal Regulations, Title 44, Chapter 1, Section 59.1 (16). Since non-levee features cannot be considered permanent structures, worst case scenario analyses (WCSAs) were performed where necessary to determine the highest possible flows on primary or secondary flooding sources. The highest computed flow was then applied to the final regulatory model. WCSAs typically included model runs with and without the non-levee feature. Additionally, several head gates exist on the main Ruby River that have the potential to affect flow rates depending on whether they are open or closed.

Elsewhere along reaches, ineffective flow was used to depict flow behind non-levee features, and mapping will be performed as if the embankment is not there. Separate model scenarios were not necessary for these situations.

The following text describes the WCSAs performed for the three models.

Beaverhead River – Upper Reach

Only one WCSA was performed for the Beaverhead River – Upper Reach model. Lateral structure LS 37144 is a 4 to 5-foot-high road and irrigation canal berm that blocks a historical flow path of the Beaverhead River, Owsley Slough. The lateral structure overtops during all modeled flood flow events. The southwest end of the structure experiences the largest amount of overtopping and was determined to be the most likely location of failure. The lateral structure was modeled two ways – intact which is the worst case for the main Beaverhead River and blown out which maximizes flow for the split reach. To model the non-levee feature as blown out, the terrain at the downstream toe of the embankment was sampled to represent natural ground at the lateral structure, and split flows were computed using the natural ground geometry.

Beaverhead River -Twin Bridges Reach

The effective FIRM panel indicates no FEMA-accredited levees in the study area. However, there is a major “non-levee feature” that separates the Beaverhead River from the town of Twin Bridges (17). The Twin Bridges non-levee feature is described as follows:

• Positioned on the right bank of the Beaverhead River extending from south of Twin Bridges downstream of XS 12610 to north of Twin Bridges at XS 5306. • Additional shorter sections of berms on the left bank exist upstream and downstream of the Highway 41 bridge. • The non-levee feature is approximately 4 to 5 feet high.

The WCSA for the Twin Bridges reach was completed by creating two model geometries – the first with the non-levee feature as intact and the second with the non-levee feature as failed. The first geometry was formatted by designating all flow area behind the non-levee feature as ineffective, so the water surface elevation in the main channel is representative of the non-levee feature holding. The second geometry assumes flow behind the non-levee feature is effective, and additional split flows were modeled through town to simulate the right bank structure’s

ALLIED ENGINEERING SERVICES, INC. 22

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS failure. A chart has been provided in Appendix H to help explain the WCSA in the Twin Bridges reach.

Ruby River

The Ruby River WCSA was more extensive given the high number of split flows and the prolific irrigation systems in the valley. The WCSA assumed the failure of four non-levee features and four open headgates leading from the main Ruby River to secondary flooding sources. Two geometry setups were created for the Ruby River. The “Worst Case Ruby” geometry file depicts all berms as intact and all headgates closed to maximize flows in the Ruby River. The “Worst Case Splits” geometry file replaces select lateral structure elevations with natural ground elevations and sets the headgates to fully open to maximize flows in secondary flood sources.

Table 4-11 provides a summary of locations of worst-case scenario analyses for all three models.

Table 4-11. Locations of Worst Case Scenario Analyses. Split Structure Source River Split(s) Structure WCSA Main WCSA Split Type River

LS 37144 Beaverhead Owsley Road and Intact Berm Failed Berm River Slough Irrigation Berm

Twin Bridges Beaverhead MainStSplit, Non-levee Ineffective flow Effective flow Non-Levee River North Split feature behind non- behind non- Feature levee feature levee feature

RBR_200 Ruby River Clear Creek Irrigation Closed Open Headgate Headgate Headgate LS 242100 Ruby River Clear Creek Irrigation Intact Berm Failed Berm Berm RBR_130 Ruby River Ruby Split 1 Irrigation Closed Open Headgate Headgate Headgate RBR_011 Ruby River Clear Creek Irrigation Closed Open Headgate Headgate Headgate LS 94600 Ruby River Mill Creek Irrigation Intact Berm Failed Berm Berm LS 4850 Mill Creek Mill Split 1 Local Intact Berm Failed Berm Road LS 27600 Ruby River Jacobs Local Intact Berm Failed Berm Slough Road RBR_010 Ruby River Ruby Split 2 Irrigation Closed Open Headgate Headgate Headgate

ALLIED ENGINEERING SERVICES, INC. 23

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

3.12 Model Calibration Historical information and gage measurements provide useful data to inform model parameters, test model sensitivity, and provide a sense of model accuracy. Several sources were investigated to assist with model calibration. 3.12.1 Historical Sources The Montana Department of Transportation Air Photo Unit and the Montana Historical Society were visited to search for aerial photographs and any other available historical flooding information. No historical flooding information for the project reaches was found.

The Montana DNRC provided photos and other information related to ice jam flooding events that occurred in 2009 on the Ruby River and 2011 on the Beaverhead River in Twin Bridges. Since ice jam modeling is beyond the scope of this hydraulic investigation, this data was not used for calibration.

Additionally, the following organizations were contacted to inquire about previous flood events. No historical flood information was found from these contacts.

• Madison County Floodplain Administrator • Twin Bridges Floodplain Administrator • Ruby Valley Conservation District • Beaverhead Watershed Committee 3.12.2 Gage Data Several gages with flow and stage information exist along the reaches modeled. As shown in Table 2-1, there are two USGS stream gages along the studied reach of the Beaverhead River and five USGS stream gages along the studied reach of the Ruby River. Elevation reference marks were surveyed at both the Beaverhead River gages and at two of the Ruby River gages. The four gages with surveyed reference marks were evaluated for model calibration. Table 3-12 provides a summary of these four USGS stream gages. Refer to Appendix H for additional calibration information.

Table 3-12. USGS Stream Gage Data Used in Model Calibration River Gage # USGS Description Model RS Beaverhead River 06023100 Beaverhead River at Twin Bridges 8157 Beaverhead River near Twin Bridges Beaverhead River 06018500 148072 MT Ruby River 06023000 Ruby River near Twin Bridges MT 11376 Ruby River below reservoir near Ruby River 06020600 287267 Alder, MT Calibration was completed at the gages shown in Table 3-12 by adjusting model parameters to better match the measured data. Measured and simulated water surface elevations within 0.5- ft were considered to be in good agreement. Calibration for the three models is described in further detail below.

ALLIED ENGINEERING SERVICES, INC. 24

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

Beaverhead River – Upper Reach

Calibration for the Beaverhead River – Upper Reach model was not able to obtain water surface elevations (WSEs) within 0.5-ft of the USGS Gage 06018500 observed elevations, and the model consistently produced WSEs below the reported gage WSEs. Model cross sections were revised by adding ineffective flow areas and raising Manning’s n values to increase WSEs, but the model could not be reasonably changed to match within 0.5-ft of the reported gage elevations. Difficulties calibrating to the gage may be due to the gage’s position immediately downstream of a bridge and a lack of surveyed bathymetric data. The average channel template applied to cross sections may not be adequate to represent scour holes or other influential bathymetry features affecting WSEs at the bridge.

Beaverhead River – Twin Bridges Reach

The Twin Bridges Reach model calibrated well to USGS Gage 06023100. The gage is located approximately 0.6 miles downstream of the Highway 41 Bridge in Twin Bridges. In general, simulations produced WSEs slightly higher than the observed WSEs, and the channel Manning’s n was lowered to 0.028 to better match the data. Lowering the channel Manning’s n value below 0.028 was not considered based on established Manning’s n ranges for rivers (18). WSEs calibrated in the range of 0.40 feet to 0.50 feet of the observed events.

Ruby River

The Ruby River also calibrated closely to the two gages with surveyed reference marks. The downstream gage, Gage 06023000, is located approximately 50 feet downstream of bridge structure RBR_020 at Seyler Lane in Reach 1. Simulations matched within 0.30 feet of observed data with a channel Manning’s n value of 0.032. The upper gage, Gage 0602060, is in the upmost portion of the Ruby just below the Ruby Reservoir. Calibration efforts at this location indicated a higher Manning’s n than the lower reaches, and a channel n value of 0.038 for Reach 5 resulted in simulations within 0.04 feet to 0.23 feet of the gage’s WSEs. The higher Manning’s n value was also justified through photographs. 3.13 Floodway Analysis The scope of work included a floodway analysis for the reach of the Beaverhead River within the corporate limits of the Town of Twin Bridges. This represents a roughly 1.5-mile stretch of the Beaverhead River. The floodway analysis was completed using the “Twin Bridges Regulatory – Levee” plan for base flood elevations. A chart has been provided in Appendix H to help explain the floodway analysis setup.

The equal conveyance reduction method was used to determine the floodway for the Beaverhead River - Twin Bridges reach. Encroachments were set so that the maximum surcharge at any given cross section was 0.5 feet per Montana guidelines. Notes on the floodway computations:

• The encroachment stations were set using the HEC-RAS program’s encroachment routines. Encroachment Methods 4 and 5 were primarily used since the methods

ALLIED ENGINEERING SERVICES, INC. 25

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

automatically adjust the encroachment stations using equal conveyance reduction to target a specified surcharge. • Negative surcharges were eliminated where possible, and any remaining negative surcharges are no more than -0.04 feet in magnitude (i.e., they can be rounded to zero). • Encroachments were, at a minimum, set at the edge of water of the 1-percent-annual- chance event (i.e., they were never left at station zero to represent no encroachment). • The equal conveyance reduction method occasionally produces a floodway that is unreasonable because of inconsistent floodway widths between cross sections. The floodway was manually adjusted at these locations using Encroachment Method 1 in the encroachment station editor. • The locations for floodway analyses were determined through coordination by Montana DNRC with community stakeholders at the start of the project. Any new floodway mapping was done where the community has a regulatory need because of anticipated development in the area. After technical review by Montana DNRC and FEMA, the floodplain mapping and floodway delineation are made available to the community as part of the Flood Risk Review phase of the project. 3.14 cHECk-RAS The cHECk-RAS computer program (19) was used as a tool to find possible errors in the HEC-RAS hydraulic model. All errors found by cHECk-RAS have either been resolved or reviewed.. The full cHECk-RAS output set can be found in Appendix I. 3.15 Other Special Hydraulic Modeling Considerations The following reaches were determined to have average flooding depths of less than one foot for significantly long reaches. Therefore, it is anticipated that these flooding sources will be mapped as shaded Zone X. No profiles have been created for these reaches. Table 3-13 highlights the flooding sources that are susceptible to shallow flooding only.

Table 3-13. Shallow Flooding Sources Flooding Source Average Depth (ft) Spring Creek 0.18 Split 3 0.17 Split 2 0.41 Split 1 0.22 California Slough (XS 85747 – 80155) 0.34 North Split 0.74 MainStSplit 0.44 Ruby Split 2 0.74 Ruby Split 1 0.33 Mill Sub Split 2 0.66 Mill Sub Split 1 0.33 Mill Split 1 0.70 Jacobs Split 1 0.84 Jacobs Slough 0.76 Clear Split 1 0.38

ALLIED ENGINEERING SERVICES, INC. 26

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

4 Floodplain Mapping FEMA technical guidance documents and other resources were reviewed during the floodplain mapping process to confirm the maps and data meet FEMA requirements. The following reference documents were consulted during development of the data set:

• Data Capture Standards Technical Reference (11) • Metadata (20) • Physical Map Revision (21) • Flood Insurance Rate Map (FIRM) Database (22)

Several maps were developed to illustrate the updates to the SFHAs in the study area. The maps are described in Sections 4.1, 4.3, and 4.4 and have been provided in the appendices. Map scale, extents, and numbering match across map sets to facilitate comparisons. 4.1 Floodplain Work Maps Floodplain mapping was produced using output from the hydraulic models and the 2017 Quantum Spatial LiDAR (3). The workmaps show the 1- and 0.2-percent-annual-chance flood event floodplain delineations and are included in Appendix B. In the Twin Bridges reach of the Beaverhead, the workmaps also show the floodway delineations. HEC-RAS’s RAS Mapper (6) was used to extract water surface elevation data, floodway extents, and raw floodplain delineations. The raw floodplain delineations were adjusted manually to provide a smoothed floodplain delineation more appropriate for small scale maps and to address backwater areas, islands, donut holes, and slivers.

The mapped floodplain and floodway topwidths of some modeled cross sections may not match the modeled topwidths. There are multiple reasons for these discrepancies which include:

• Removal of small islands (<625 SF) from mapping. • Removal of hydraulically disconnected areas from mapping. • Areas where engineering judgement was used to extend, taper, or trim the floodplain boundary to create a more realistic floodplain delineation.

Engineering judgment was used during the mapping process in many locations to create realistic floodplain and floodway extents. Some of the common scenarios where engineering judgment was used include:

• Extending the floodplain boundary where the raw floodplain delineation was cut off between the limits of two cross sections (i.e. the raw floodplain delineation shows a straight line through an oxbow). • Extending the floodplain boundary where the raw flooding extents terminate but the topography shows a gradient which would cause the floodwater to continue down valley. • Trimming the floodplain boundary to remove floodplain slivers at irrigation canals. • Trimming or extending the floodplain boundary in backwater areas.

ALLIED ENGINEERING SERVICES, INC. 27

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

4.2 Effective Tie-In Locations There are no flooding sources or locations that require tie-in to effective SFHAs. The mapping along the Beaverhead River extends beyond the effective mapping in Twin Bridges. 4.3 Changes Since Last FIRM Mapping – 1-Percent-Annual-Chance Flood Event Comparison The Changes Since Last FIRM (CSLF) dataset compares the effective 1-percent-annual-chance flood event on the 1986 FIRM for the Town of Twin Bridges to the 1-percent-annual-chance flood event proposed by this study. For the other reaches of this study, the CSLF dataset only shows the 1-percent-annual-chance flood event for this study since these reaches have not been previously studied by FEMA. The CSLF work maps are included in Appendix J. FEMA’s Guidance for Flood Risk Analysis and Mapping: Changes Since Last FIRM (23) document was used to guide creation of the product. 4.4 Changes Since Last FIRM Mapping – Floodway Comparison The CSLF – Floodway Comparison maps are in Appendix K. Since the scope of this study only included floodway analysis within the incorporated limits of the Town of Twin Bridges, Floodway Comparison maps are limited to the Twin Bridges reach of the Beaverhead River. 4.5 Floodplain Boundary Standard Audit The Floodplain Boundary Standard (FBS) audit compares the water surface elevations from a hydraulic model to best available terrain data to verify that the floodplain delineations are accurate. The FBS process outlined in FEMA’s Guidance for Flood Risk Analysis and Mapping: Floodplain Boundary Standards guidance (24) was followed to complete the FBS audit. The Beaverhead River and Ruby River study area within Madison County is designated as a Risk Class C. The FBS guidance document states that 85% of the audit points must have their computed water surface elevation and the ground elevation within ± 1.0 foot to meet the delineation reliability standards for an Enhanced level study categorized as a Risk Class C.

The FBS audit’s pass rate for the reaches was initially computed without excepting any points. If the 85% pass rate was not achieved, points were excluded for reasons including backwater, structures, flood continuation, or confluences. Reaches that required point exclusions included Mill Creek, Split 4, and the Beaverhead River at Twin Bridges. 4.6 Flood Depth Grids Flood depth grids were created for the 10, 4, 2, 1, 1 plus- and 0.2-percent-annual chance flood events to show the inundation depths across the study area. FEMA’s Guidance for Flood Risk Analysis and Mapping: Flood Depth and Analysis Grids (25) was referenced to create the dataset. The digital datasets that accompany this report include the flood depth grids. The depth grids represent the raw output from RAS Mapper and have not been manipulated for use as regulatory level products per Exhibit A of Contract WO-AESI-184.

ALLIED ENGINEERING SERVICES, INC. 28

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

4.7 Water Surface Elevation Grids Water surface elevation grids were created for the 10, 4, 2, 1, 1 plus- and 0.2-percent-annual chance flood events to show the water surface elevations across the study area. FEMA’s Guidance for Flood Risk Analysis and Mapping: Flood Depth and Analysis Grids (25) was referenced to create the dataset. The digital datasets that accompany this report include the water surface elevation grids. The water surface elevation grids represent the raw output from RAS Mapper and have not been manipulated to be used as regulatory level products per Exhibit A of Contract WO-AESI-184.

5 Flood Insurance Study The Flood Insurance Study (FIS) Text, Floodway Data Tables, and Water Surface Elevation Profiles were created for the Flood Insurance Study and are described in Sections 5.1, 5.2, and 5.3, respectively.

The following references were used to create the products:

• Technical Reference: FIS Report (26) • Guidance for Flood Risk Analysis and Mapping: Flood Insurance Study Report (27) 5.1 FIS Text The relevant FIS tables have been populated with data from this study. The FIS information is in Appendix M. 5.2 Floodway Data Tables The Floodway Data Table for the Twin Bridges reach of the Beaverhead River is in Appendix N. 5.3 Water Surface Elevation Profiles The water surface elevation profiles show the 10-, 4-, 2-, 1-, and 0.2-percent-annual-chance flood events and the 1-percent-plus-annual-chance event. These are included in Appendix O.

6 REFERENCES 1. FEMA. Flood Insurance Rate Map, Town of Twin Bridges, Montana, Community Panel Number 3000450001B, Effective Date July 3, 1986. 1986.

2. Morrison-Maierle. Survey Report: Jefferson River Watershed, Phase 1, Broadwater, Gallatin, Jefferson, and Madison Counties, Mapping Activity Statement No. 2017-04. 2017.

3. Quantum Spatial. Jefferson River Watershed, Montana, LiDAR Technical Data Report. 2018.

4. Michael Baker International. Jefferson River Watershed Hydrologic Analysis, Gallatin and Madison Counties, MT. 2018.

5. US Census Bureau. American Fact Finder. [Online] 2019. https://factfinder.census.gov/faces/nav/jsf/pages/community_facts.xhtml. Accessed on May 6, 2019.

6. USACE. HEC-RAS 5.0.7. s.l. : USACE, 2019.

ALLIED ENGINEERING SERVICES, INC. 29

BEAVERHEAD RIVER, RUBY RIVER AND SPLITS

7. ESRI. ArcMap 10.5.1. 2017.

8. CivilGEO. GeoHECRAS.

9. USACE. HEC-RAS River Analysis System User's Manual. s.l. : USACE, 2016.

10. FEMA. FEMA Policy Standards for Flood Risk Analysis and Mapping, FEMA Policy #FP 204- 078-1 (Rev 7). 2018.

11. —. Data Capture Technical Reference. 2018.

12. —. Guidance for Flood Risk Analysis and Mapping: Data Capture - General. 2017.

13. —. Data Capture - Workflow Details. 2018.

14. USACE. HEC-RAS Hydraulic Reference Manual Version 5.0. Davis : s.n., 2016. 15. —. HEC-RAS 2D Modeling User's Manual . 2016.

16. US Government Printing Office. Code of Federal Regulations: Title 44, Chapter 1, Section 59.1. 2006.

17. FEMA. Guidance for Flood Risk Analysis and Mapping: Levees. 2018.

18. Chow, Ven Te. Open-Channel Hydraulics. s.l. : The Blackburn Press, 1959.

19. FEMA. cHECk-RAS Version 2.0.1. 2013. 20. —. Guidance for Flood Risk Analysis and Mapping: Metadata. 2018.

21. —. Guidance for Flood Risk Analysis and Mapping - Physical Map Revision (PMR). s.l. : FEMA, 2019.

22. —. Guidance for Flood Risk Analysis and Mapping: Flood Insurance Rate Map (FIRM) Database. 2018.

23. —. Guidance for Flood Risk Analysis and Mapping: Changes Since Last FIRM. 2018.

24. —. Guidance for Flood Risk Analysis and Mapping: Floodplain Boundary Standards. 2015.

25. —. Guidance for Flood Risk Analysis and Mapping: Flood Depth and Analysis Grids. 2018.

26. —. Flood Insurance Study (FIS) Report Technical Reference: Preparing FIS Reports. 2018.

27. —. Guidance for Flood Risk Analysis and Mapping: Flood Insurance Study (FIS) Report. 2016.

ALLIED ENGINEERING SERVICES, INC. 30