1.0 introductiondeq.mt.gov/portals/112/land/hardrock/active amendments... · 2019-11-18 · updated...

File No.: NB101-00045/44-A.01 1 of 14 Cont. No.: NB19-00559

August 8, 2019

Mr. Matt Wolfe

Environmental Manager

Sibanye Stillwater (East Boulder)

P.O. Box 1227

Big Timber, Montana

USA, 59011

Knight Piésold Ltd.

1650 Main Street West

North Bay, Ontario

Canada, P1B 8G5

T +1 705 476 2165

E [email protected]

www.knightpiesold.com

Dear Matt,

RE: East Boulder Mine Stage 6 TSF Expansion - East Boulder River Modelling Assessment

1.0 INTRODUCTION

Stillwater Mining Company (SMC) is in the process of permitting the Stage 6 Tailings Storage Facility (TSF)

expansion at the East Boulder Mine. Knight Piésold Ltd. (KP) prepared the design report for the

Stage 6 TSF expansion (KP, 2019a). The Environmental Assessment for the Stage 6 TSF was initiated

during the first quarter of 2019 by the Montana Department of Environmental Quality (MDEQ) and

Custer Gallatin National Forest (CGNF).

The downstream embankment raise for the Stage 6 TSF expansion results in the downstream embankment

toe being constructed closer to the East Boulder River. The embankment section that is closest to the

East Boulder River was identified as a critical section during the development of the Stage 6 TSF expansion

design. Erosion or potential valley wall (riverbank) instability was identified as a potential risk to the

Stage 6 TSF expansion at the critical section. As a result, an assessment of potential erosion and riverbank

instability was completed to confirm the embankment stability and identify monitoring and mitigation

measures (KP, 2019a).

SMC received an information request from the MDEW and CGNF on May 3, 2019 (Environmental

Resources Management (ERM), 2019) requesting that a HEC-RAS (USACE, 2019) model be completed

to confirm the results of the previous analyses. Additional information on the stream flow estimates that

were obtained from the United States Geological Survey (USGS) StreamStats (USGS, 2017) program was

also requested.

This letter provides the following:

A brief summary of the previously completed riverbank erosion assessment

Additional information on the flow estimates from StreamStats

A summary of the HEC-RAS hydrologic modelling

Updated riverbank and riverbed erosion estimates based on the HEC-RAS hydrologic modelling

The site plan and critical section area are illustrated on Figure 1.

August 8, 2019 2 of 14 NB19-00559

Figure 1 Site Plan with Monitoring Stations and Photo Locations (Presented in Appendix C)

2.0 PREVIOUS ASSESSMENT

The previous riverbank erosion and stability assessment completed by KP (KP, 2019a) utilized Manning’s

open channel flow equation to estimate flow depths and velocities in the East Boulder River at the critical

section during select return period flood events. The peak flows for the 100, 200, and 500-year flood events

were obtained from StreamStats (USGS, 2017). The 1,000 and 10,000 year peak flows were extrapolated

from the StreamStats values. A Manning’s n value of 0.055, representative of mountain streams with no

vegetation in the channel was selected for the analysis.

The estimated range of flow depths and velocities from Manning’s equation based on the average flow

estimates from StreamStats (USGS, 2017) for the 100 to 10,000 year return period events were as follows:

Flow depth: approximately 3 to 4 ft

Velocity: approximately 10 to 13 fps

It was determined that the estimated flood depths would be fully contained within the existing river channel.

The estimated flow velocities would result in the mobilization of a portion of the riverbed and bank material,

however the largest boulder fractions would remain. Exposure of additional boulders in the eroded bed and

bank would result in additional armoring that would tend to limit the extent of erosion. The erosion/channel

migration potential was concluded to be low based on the observed conditions and estimated flow

August 8, 2019 3 of 14 NB19-00559

conditions during the return period flood events. The assessment concluded that the Stage 6 TSF

embankment will remain stable during operations and in the long-term after closure, even if extreme design

storm events with associated high stream flows should occur within the adjacent East Boulder River

channel.

3.0 STREAMSTATS

StreamStats is a web-based Geographic Information System (GIS) application created by USGS (with the

Environmental Systems Research Institute, Inc. (ESRI)). Users of the application can obtain streamflow

statistics, basin characteristics and other descriptive information for USGS data-collection stations and

user-selected ungaged sites.

The application is available for use in most of the United States and uses data collected nationwide. The

Flood Frequency Map for Montana (which includes a portion of Wyoming in the Yellowstone and Little

Missouri River Basins) is divided into eight hydrologic regions as illustrated on Figure 2. The hydrologic

regions are defined as areas having similar climate, vegetation, and topography. The East Boulder River is

located within the Upper Yellowstone-Central Mountain hydrologic region.

There are 537 streamflow-gaging stations in or near Montana and approximately 90 gaging stations are

used in the regression analyses for the Upper Yellowstone-Central Mountain Hydrologic Region.

Five gaging stations are located in the Boulder River watershed; including one on the East Boulder River,

two on the West Boulder River, and two on the main reach of the Boulder River as shown on Figure 3.

These stations have been collecting data for many decades. For example, the period of record for USGS

Station Number 06197500 (Boulder River near Contact MT) is 1910-1916, 1929, 1951-1969, 1971-1975,

1982-1983, and for USGS Station Number 06198000 (East Boulder River near McLeod) is from

approximately 1907 to 1983.

StreamStats uses regression equations to estimate peak-flow frequencies (i.e. peak flow magnitudes, in

cfs, associated with annual exceedance probabilities of 66.7, 50, 42.9, 20, 10, 4, 2, 1, 0.5, and 0.2 percent)

at ungaged sites. The annual exceedance probabilities correspond to 1.5, 2, 2.33, 5, 10, 25, 50, 100, 200,

and 500-year recurrence intervals, respectively. The regression equations were developed using

generalized least-squares regression or weighted least squares regression (Sando, et. al., 2015). The

regression equations used for Montana are provided in Appendix A (Table A.1). The regression equations,

developed for each of the hydrologic regions, take into account the hydrologic characteristics of each region

including the percentage of the basin that is forest, the percentage of the basin that is above 5,000 feet

elevation, and the percentage of the basin with slopes greater than 30 percent.

The use of StreamStats is deemed to be suitable for the East Boulder River flooding assessment due to

the local stream gaging stations, extensive historical flows records and the regression analysis used to

estimate flows for ungaged streams.

The estimated flows from StreamStats were compared with flow data collected by SMC at monitoring

station EBR-003. The peak flows recorded in May, June and July from 2001 to 2019 at EBR-003 range

from approximately 210 to 760 cfs. These peak flows recorded during the freshet period are similar to the

StreamStats estimates ranging from the 2-year to 50-year return period events (flows of

approximately 220 to 810 cfs).

August 8, 2019 4 of 14 NB19-00559

Figure 2 Montana Hydrologic Regions (USGS, 2019)

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Figure 3 Boulder River Watershed Stream Gage Locations

4.0 HEC-RAS ANALYSIS

4.1 GENERAL

A two-dimensional (2D) HEC-RAS model was developed to estimate stage and flow velocities at select

locations along the East Boulder River including the critical section that is located at a meander in the

East Boulder River as shown on Figure 1. The 2D HEC-RAS modelling approach utilizes the Full

Momentum (Saint Venant) equation to solve for unsteady river flow. The East Boulder River model was

developed with several datasets including topography (i.e., LiDAR) and land use classification. A graphical

August 8, 2019 6 of 14 NB19-00559

summary of the model setup is provided in Appendix B (Figure B.1) and each model input is described in

detail below.

4.2 TOPOGRAPHY

LiDAR data from the 2016 United States Geological Survey (USGS, 2016), and the recent

2018 survey (Stillwater Mining Company (SMC), 2018) were utilized to create the topographic terrain model

in HEC-RAS. Each LiDAR surface was merged into one composite surface utilizing the blend tool

in ArcMap (Version 10.7). The composite LiDAR surface was imported into HEC-RAS and forms the basis

of water routing within the 2D model.

The LiDAR surface was modified in HEC-RAS to account for the river bathymetry as LiDAR does not

penetrate water. A total of 80 cross sections were defined along the East Boulder River in the HEC-RAS

model and modified to incorporate river bathymetry. The LiDAR ground surface along the river channel was

lowered by approximately 2 ft. in areas where the LiDAR surface did not receive a return signal. The 2 ft.

offset was selected based on the review of the recorded water depths in the East Boulder River hydrometric

monitoring dataset at the time of the 2018 LiDAR survey (May 2018). The bathymetry generated from the

modified sections was used to update the topographic terrain model in HEC-RAS.

4.3 LAND USE CLASSIFICATION

Based on a review of the aerial imagery, site photographs and observed site conditions, land use

classification for the model was divided as follows:

Riverbed and Riverbank Sediments - The riverbed consists largely of cobble sized clasts or larger

particles throughout the model domain. Riverbank materials outside of the high water mark along the

southwest side of the river channel consist of Glacial Till (dense gravel and sand with frequent cobbles

and boulders), riverbank materials along the northeast side of the river channel consists

of Alluvium (compact to dense Sand and Gravel with frequent cobbles) (KP, 2019b).

Forested Areas - Consists largely of densely populated coniferous trees, fallen trees and shrub

undergrowth.

Modified Terrain - Consists of elements of the mine infrastructure and TSF including embankments,

roads, and stockpiles.

Select photographs illustrating the river channel conditions at several locations along the East Boulder River

are provided in Appendix C. The general locations of the photographs are shown on Figure 1.

4.4 BOUNDARY CONDITIONS AND STREAMSTATS DISCHARGE RATES

The HEC-RAS model includes two boundary conditions. The external upgradient boundary condition

consists of a user-defined flow hydrograph that defines the water entry into the model domain. The external

downgradient boundary condition consists of a normal depth function that allows for water to exit the model.

The upstream boundary condition was positioned upstream of the TSF to minimize influence on the model

results and to incorporate the dynamics of the braided portion of the East Boulder River near the southeast

boundary of the TSF. The upstream flow hydrograph boundary condition was defined by the peak discharge

values estimated from StreamStats.

August 8, 2019 7 of 14 NB19-00559

The average estimated flows in of the East Boulder River for the return period flood events that were

considered for the assessment are summarized in Table 1. Peak flows for the 100, 200 and 500-year return

periods were estimated using StreamStats. The 1,000 year and 10,000 year peak flows were extrapolated

from the StreamStats values.

Table 1 Summary of Average Flood Flows

Flood Return Period

(years)

Peak Flow

(cfs)

100 959

200 1,120

500 1,340

1,000 1,431

10,000 1,888

NOTES: 1. REFERENCED FROM KP, 2019A.

2. PEAK FLOWS UTILIZED AS FLOW HYDROGRAPH VALUES FOR MODEL RUNS.

4.5 MODEL CALIBRATION

SMC provided KP a dataset of detailed streamflow measurements for several locations along the

East Boulder River between 2001 and 2019 (SMC, 2019). The dataset included near monthly profiling of

the East Boulder River from May through November for each year. Profiling consisted of measuring the

water depth and velocity at increments along the river, as well as the stage (if the staff gage was installed).

This dataset allowed for the calibration of both stream velocity and water surface elevation (WSE) with the

results from HEC-RAS.

The data used to calibrate the model was collected at monitoring station EBR-003, located

approximately 1,500 ft. downstream of the critical section area as shown on Figure 1. The discharge and

maximum velocity measurements recorded at EBR-003 are provided in Appendix B (Figure B.2).

To calibrate the HEC-RAS model, the flow hydrograph (upstream boundary condition) was adjusted to

include a range of discharge flow rates similar to the range measured at EBR-003 (10 to 760 cfs). Multiple

model runs were then completed using a range of Manning’s n values for the Riverbed and Riverbank

Sediments in order to determine the most appropriate Manning’s n value to represent the flow conditions

within the East Boulder River. Manning’s n values for Forested Areas and Modified Terrain were kept

constant at 0.10 and 0.04, respectively. The results of each calibration run are illustrated in

Appendix B (Figures B.2, B.3 and B.4) and include a summary of the simulated velocity and discharge

measurements, observed velocity versus modelled velocity and stage versus discharge, respectively. A

root mean square error (RMSE) statistical calculation was completed for each model run to illustrate

calibration of the modelled velocity under varying surface roughness coefficients (Appendix B.5) and river

stage (Appendix B.6). The calibration analyses determined that a Manning’s n value of 0.055 best simulated

the observed velocities at high velocity/discharge levels. Manning’s n values of 0.04 and 0.08 were

August 8, 2019 8 of 14 NB19-00559

evaluated as sensitivity analyses to illustrate the potential range in flow conditions that may occur along the

river channel.

4.6 RESULTS

A summary of the maximum velocities estimated in HEC-RAS for each flood event and the corresponding

estimated river depth at the critical section area is presented in Table 2. Table 2 includes the results of the

previous assessment using Manning’s Open Channel Equation for comparison purposes.

The modelled river velocity and depth is shown in plan and section on Figures 4, 5 and 6 for 100-year,

1,000 year and 10,000 year events, respectively. The estimated river velocity adjacent to the riverbanks

ranges from approximately 1 to 8 fps and the estimated river velocity at the center of the river channel

ranges from approximately 8 to 11 fps.

In general, the modelled depth of flow was approximately 6 inches higher than the estimated depth of flow

using Manning’s open channel equation (KP, 2019a). The corresponding modelled velocity was

approximately 1.5 fps less than along the criterial meander as compared to values determined using

Manning’s open channel equation. The highest velocities along the section correlate to the deepest part of

the channel, with a reduction in velocity towards the riverbanks.

Figure 4 100-Year Return Flood - Modelled River Velocity and Water Depth

August 8, 2019 9 of 14 NB19-00559

Figure 5 1,000-Year Return Flood - Modelled River Velocity and Water Depth

Figure 6 10,000 - Year Return Period Flood - Modelled River Velocity and Water Depth

August 8, 2019 10 of 14 NB19-00559

4.6.1 SENSITIVITY ANALYSIS

A sensitivity analysis on the roughness parameter (Manning’s n) was completed to illustrate the potential

effects of variation in stream channel roughness. Model runs were completed using a range of

Manning’s n values (0.04 to 0.08). The results are presented in Table 3. The lower Manning’s n values

resulted in lower flow depths and higher velocities and the higher Manning’s n values resulted in higher

flow depth and lower velocities, as expected. The range in estimated flow depths and velocities is minimal

and the selected values for the roughness parameter are considered reasonable.

5.0 RIVERBANK EROSION POTENTIAL

The maximum particle sizes that may be transported from the riverbed and riverbank were estimated using

Shield’s method (American Society of Civil Engineers (ASCE), 2008; Shields, 1936) which considers the

critical shear stress of the riverbed and riverbank material, the specific gravity of the particles and the

estimated flow depth and slope of the river. Similar to the previous assessment, a critical shear stress value

of 0.054 was assumed for a riverbed consisting of coarse cobbles (USGS, 2013).

The estimated maximum particle sizes that will be transported during the return period flood events ranges

from 11 to 17 inches and are summarized in Table 2. These are the median particle sizes in a well-sorted

bed material mixture that would be at the state of incipient motion during the indicated flood events. Larger

lag boulders that are not part of the sorted mixture would remain stable or would move slightly due to

undermining of adjacent material.

It is noted that the estimated riverbed slope in the previous assessment was 4.8% and the subsequent

assessment included a 2.6% riverbed slope to be consistent with the HEC-RAS model. The reduced

riverbed slope resulted in a reduction in the estimated particle size that would be transported during the

return period events.

Briaud (2013) developed an empirical soil erosion chart to estimate the potential erosion rate of soil and

rock based on velocity. The East Boulder riverbed and riverbank is composed mainly of coarse gravel,

cobbles and boulders. The materials are classified as Low Erodibility to Very Low Erodibility on the soil

erosion chart shown on Figure 7.

August 8, 2019 11 of 14 NB19-00559

Figure 7 Soil Erosion Chart (Briaud, 2013)

The range of modelled river velocity along the critical section was estimated to be

approximately 2 to 8 ft/s (0.6 to 2.4 m/s) for the riverbanks, and 8 to 10.8 ft/s (2.4 to 3.3 m/s) for the main

channel. Using Briaud’s soil erosion chart (Briaud, 2013) this relates to a bank erosion rate of 0.1 mm/hr to

1 mm/hr, and a channel erosion rate of 1 mm/hr to 3 mm/hr.

If the peak flow during the 10,000-year event was conservatively assumed to occur for a 24-hour period,

the potential lateral erosion of the riverbank during the 10,000-year could range from

approximately 0.1 to 1 inches along the river bank, and 1 to 3 inches along the riverbed. It is noted that the

estimated erosion is smaller than the size of particles that could potentially be mobilized during the return

period events and the erosion magnitude would be similar to size of particles that could be potentially

mobilized.

6.0 CONCLUSIONS

The following conclusions are provided based on the results of the HEC-RAS hydrologic modelling:

The HEC-RAS model results are similar to the previous assessment using Manning’s open channel

equation (KP, 2019a), confirming the earlier velocity, flow depth and erosion estimates.

A Manning’s n of 0.055 for the riverbed and riverbank material was determined to best represent the

observed velocity measurements at EBR-003.

The HEC-RAS velocity profile results illustrate that the highest velocities would occur towards the center

of the river channel and lower velocities would occur along the riverbank during the return period flood

events.

The presence of large boulders in the riverbed and banks provides a buffer against lateral and vertical

erosion, as the exposure of additional boulders by erosion of finer gravel and cobbles will provide an

armoring effect.

The additional hydraulic modelling and erosion analysis confirms that erosion/channel migration

potential is low during the return period flood events and that the riverbank will remain stable during

operations and following closure.

Riverbed and Riverbank material shaded in green

August 8, 2019 13 of 14 NB19-00559

References:

American Society of Civil Engineers (ASCE), 2008. Sedimentation Engineering. Processes,

Measurements, Modelling and Practice. ASCE Manuals and Reports on Engineering Practice.

No. 110.

Briaud, 2013. Geotechnical Engineering: Unsaturated and Saturated Soils. John Wiley and Sons,

New York.

Environmental Resources Management (ERM), 2019. Technical Memorandum 1. Subject: Review and

Findings for Analysis of River Avulsion/Erosion. May 3.

Knight Piésold Ltd. (KP), 2019a. Detailed Design for Stage 6 TSF Expansion. April 12. North Bay, Ontario.

Ref. No. NB101-44/38-1, Rev 4.

Knight Piésold Ltd. (KP), 2019b. Geological and Geotechnical Site Conditions. May 1. North Bay, Ontario.

Ref. No. NB101-45/44-8, Rev 0.

Sando, R., Sando, S.K., McCarthy, P.M. and Dutton, D.M., 2015. Methods for estimating peak-flow

frequencies at ungaged sites in Montana based on data through water year 2011: Chapter F in

Montana StreamStats. Scientific Investigations Report 2015-5019-F. Prepared in cooperation with

the Montana Department of Natural Resources and Conservation.

Sando, McCarthy, and Dutton, 2016. Temporal trends and stationarity in annual peak flow and peak-flow

timing for selected long-term streamflow-gaging stations in or near Montana through water year

2011: Chapter B in Montana StreamStats. Scientific Investigations Report 2015-5019- B. Prepared

in cooperation with the Montana Department of Transportation and Montana Department of Natural

Resources and Conservation.

Shields, A., 1936. Anwendung der Äenlichkeits-Mechanik und der Turbulenzforschung auf die

Geschiebebewegung, Mitteilungen der Preussische Versuchsanstalt für Wasserbau und Schiffbau,

Berlin, Heft 26. Translation by W.P. Ott and J. C. van Uchelen. Soil Conservation Service, California

Institute of Technology, Pasadena.

Stillwater Mining Company (SMC), 2018. LiDAR Dataset. Acquired May 2018. Received June 2018.

Stillwater Mining Company (SMC), 2019. EBR Flow Data (years 2001 to 2019). Excel Spreadsheets.

Received July 10, 2019 from Marty Talley.

United States Army Corps of Engineers (USACE), 2019. HEC-RAS. Version. 5.07. March.

Hydrologic Engineering Center, River Analysis System.

United States Geological Society (USGS), 2013. Scientific Investigations Report 2008-5093. Retrieved

from: http://pubs.usgs.gov/sir/2008/5093. January 10.

United States Geological Society (USGS), 2016. 1 meter Digital Elevation Models (DEMs) UGSG National

Map 3DEP Collection. Acquisition date August 6 to 11, 2016. Retrieved from

https://catalog.data.gov/dataset/usgs-ned-one-meter-x56y504-mt-stillwater-2016-img-2019

(assessed Downloaded June 18, 2019).

August 8, 2019 14 of 14 NB19-00559

United States Geological Society (USGS), 2017. StreamStats. Retrieved from:

https://water.usgs.gov/osw/streamstats/index.html (assessed April 20, 2017).

United States Geological Society (USGS), 2019. Water Resources of the United States, Montana,

Statewide Rural.

/sf

Flow(2) Depth of Flow Velocity Slope of ChannelEstimated Particle Size for Sediment

Transport(1)Depth of Flow Velocity Slope of Channel

Estimated Particle Size for Sediment

Transport(1)

(years) (cfs) (ft.) (fps) (%) (in) (ft.) (fps) (%) (in)

100 959 3.2 8.8 2.6% 11 2.7 10.2 4.8% 17

200 1,120 3.5 9.3 2.6% 12 2.9 10.7 4.8% 19

500 1,340 3.9 9.8 2.6% 14 3.2 11.3 4.8% 21

1,000 1,431 4.1 10.0 2.6% 14 3.4 11.6 4.8% 22

10,000 1,888 4.7 10.8 2.6% 17 3.9 12.7 4.8% 25

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Tables\[Table 2 - Summary of Water Depth and Flows.xlsx]Table 2

NOTES:

TABLE 2

EAST BOULDER RIVER MODELLING ASSESSMENT

EAST BOULDER MINESTILLWATER MINING COMPANY

Flood Return Period

Print Aug-08-19 12:50:28

2. FLOOD RETURN PERIOD FLOW DETERMINED USING PEAK FLOW STATISTICS FROM STREAMSTATS (AVERAGE INTERVAL)

Manning's Open Channel Flow Equation (KP, 2019)HEC-RAS Model

1. MAXIMUM ESTIMATED TRANSPORTED PARTICLE SIZE CALCULATED USING SHIELDS (SHIELDS, 1936).

SUMMARY OF MODELLED FLOOD FLOWS, DEPTHS AND VELOCITIES, AND SEDIMENT TRANSPORT PARTICLE SIZES

0 08AUG'19 SBFISSUED WITH LETTER NB19-00559 CNHDATE DESCRIPTION PREP'D RVW'DREV

Page 1 of 1

Depth of Flow Velocity Depth of Flow Velocity Depth of Flow Velocity

(years) (cfs) (ft.) (fps) (ft.) (fps) (ft.) (fps)

100 959 3.2 8.8 2.8 11.2 3.8 6.8

200 1,120 3.5 9.3 3.1 11.4 4.1 7.2

500 1,340 3.9 9.8 3.4 12.0 4.5 7.6

1,000 1,431 4.1 10.0 3.6 12.2 4.7 7.8

10,000 1,888 4.7 10.8 4.2 13.2 5.4 8.4

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Tables\[Table 3 - Summary Sensitivity Analysis.xlsx]Table 3

NOTES:

1. FLOOD RETURN PERIOD FLOW DETERMINED USING PEAK FLOW STATISTICS FROM STREAMSTATS (AVERAGE INTERVAL)

Base Case (n = 0.055) Lower Bound (n = 0.04) Upper Bound (n = 0.08)Flow(1)

STILLWATER MINING COMPANY

TABLE 3

SUMMARY OF SENSITIVITY ANALYSIS

Flood Return Period

Print Aug-08-19 12:52:03

EAST BOULDER RIVER MODELLING ASSESSMENT

EAST BOULDER MINE


Page 1 of 1

August 8, 2019 NB19-00559

APPENDIX A

StreamStats Regression Equations

(Page A-1)

Regression equation for indicated Q AEP

Number of streamflow-gaging

stations (n )1σ δ

2, in log units MVP , in log units SEP , in percent SEM , in percent Pseudo R 2, in percent

Q 66.7 = 2.75 A 0.825 (E 6000 + 1)0.243 87 0.159 0.167 119.2 115.2 81.6

Q 50 = 4.73 A 0.802 (E 6000 + 1)0.194 91 0.144 0.151 111.0 107.3 81.3

Q 42.9 = 6.40 A 0.792 (E 6000 + 1)0.158 91 0.130 0.136 103.0 99.5 81.7

Q 20 = 19.9 A 0.759 (E 6000 + 1)0.028 91 0.092 0.098 82.4 79.4 82.6

Q 10 = 41.1 A 0.741 (E 6000 + 1)-0.052 91 0.075 0.081 73.0 70.0 82.9

Q 4 = 84.5 A 0.722 (E 6000 + 1)-0.125 91 0.067 0.072 68.4 65.1 82.2

Q 2 = 131 A 0.708 (E 6000 + 1)-0.166 91 0.065 0.071 67.7 64.1 81.0

Q 1 = 189 A 0.695 (E 6000 + 1)-0.199 91 0.067 0.073 69.0 65.2 79.3

Q 0.5 = 262 A 0.683 (E 6000 + 1)-0.227 91 0.071 0.078 71.6 67.4 77.2

Q 0.2 = 384 A 0.668 (E 6000 + 1)-0.258 91 0.079 0.088 77.0 72.4 73.7

NOTES:

TABLE A.1

STILLWATER MINING COMPANYEAST BOULDER MINE

EAST BOULDER RIVER MODELLING ASSESSMENTREGRESSION EQUATIONS FOR ESTIMATING PEAK-FLOW FREQUENCIES AT UNGAGED SITES IN MONTANA

Upper Yellowstone-Central Mountain Hydrologic Region

Print Aug-08-19 11:17:05

1. [QAEP, PEAK-FLOW MAGNITUDE, IN CUBIC FEET PER SECOND, FOR ANNUAL EXCEEDANCE PROBABILITY (AEP) IN PERCENT; N, NUMBER OF STREAMFLOW-GAGING STATIONS USED IN DEVELOPING REGRESSION EQUATIONS FOR INDICATED HYDROLOGIC REGION; ΣΔ2, MODEL ERROR VARIANCE; MVP, MEAN VARIANCE OF PREDICTION; SEP, MEAN STANDARD ERROR OF PREDICTION; SEM, MEAN STANDARD ERROR OF MODEL; PSEUDO R2, PSEUDO COEFFICIENT OF DETERMINATION; A, CONTRIBUTING DRAINAGE AREA, IN SQUARE MILES; P, MEAN ANNUAL PRECIPITATION, IN INCHES; F, PERCENTAGE OF BASIN THAT IS FOREST; E5000, PERCENTAGE OF BASIN ABOVE 5,000 FEET ELEVATION; SLP30, PERCENTAGE OF BASIN WITH SLOPE GREATER THAN 30 PERCENT; ETSPR, MEAN SPRING (MARCH–JUNE) EVAPOTRANSPIRATION, IN INCHES PER MONTH; E6000, PERCENT OF BASIN ABOVE 6,000 FEET ELEVATION].

2. THE NUMBER OF STREAMFLOW-GAGING STATIONS USED IN THE Q66.7 REGRESSION EQUATION FOR A REGION MIGHT DIFFER FROM THE NUMBER OF STREAMFLOW-GAGING STATIONS USED IN ALL OTHER REGRESSION EQUATIONS IN THAT REGION BECAUSE OF STREAMFLOW-GAGING STATIONS WITH UNREPORTED Q66.7 VALUES, WHICH IS DISCUSSED FURTHER IN SANDO, MCCARTHY, AND DUTTON (2016).

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Appendix A - StreamStats Regression Table\[Appendix A - StreamStats Regression Table.xlsx]Appendix Table A.1

3.TABLE ADAPTED FROM: SANDO, R., SANDO, S.K., MCCARTHY, P.M. AND DUTTON, D.M., 2015. METHODS FOR ESTIMATING PEAK-FLOW FREQUENCIES AT UNGAGED SITES IN MONTANA BASED ON DATA THROUGH WATER YEAR 2011: CHAPTER F IN MONTANA STREAMSTATS. SCIENTIFIC INVESTIGATIONS REPORT 2015-5019-F. PREPARED IN COOPERATION WITH THE MONTANA DEPARTMENT OF NATURAL RESOURCES AND CONSERVATION.


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A-1 of 1

August 8, 2019 NB19-00559

APPENDIX B

HEC-RAS Model Setup and Calibration

(Pages B-1 to B-6)

Riverbed/Riverbank Sediments

Forested Areas Modified Terrain

CAL-1 0.065 0.1 0.04 0.8

CAL-2 0.055 0.1 0.04 0.69

CAL-3 0.06 0.1 0.04 0.78

CAL-4 0.04 0.1 0.04 1.15

CAL-5 0.08 0.1 0.04 1.07

NOTES:1. RMSE CALCULATED USING THE DIFFERENCE BETWEEN THE MODELED AND OBSERVED DATASETS. OBSERVED DATA REGRESSION CALCULATED USING THE FOLLOWING REGRESSION: VELOCITY (ft/s) = 0.4728X0.4401.

Model Trial

Manning’s n Velocity Root Mean Square Error (ft/s)

TABLE B.1


EAST BOULDER RIVER MODELLING ASSESSMENTMODELLED VELOCITY CALIBRATION STATISTICS

Print Aug-08-19 11:40:15

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Appendix B - HEC-RAS Results\[Fig B.2, B.3, B.4, Tab B.1, B.2 - EBR-003 - Calibration.xlsx]Table B.1


Page 1 of 1

B-1 of 6

Riverbed/Riverbank Sediments

Forested Areas Modified Terrain

CAL-1 0.065 0.1 0.04 0.81

CAL-2 0.055 0.1 0.04 0.85

CAL-3 0.06 0.1 0.04 0.84

CAL-4 0.04 0.1 0.04 0.95

CAL-5 0.08 0.1 0.04 1.03

NOTES: 1. RMSE CALCULATED USING THE DIFFERENCE BETWEEN THE MODELED AND OBSERVED DATASETS. OBSERVED DATA REGRESSION CALCULATED USING THE FOLLOWING REGRESSION: STAGE (ft)= 5E-08X3 - 5E-05X2 + 0.0168X + 6141.4.

Model Trial

Manning’s n Stage Root Mean Square Error (ft.)

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Appendix B - HEC-RAS Results\[Fig B.2, B.3, B.4, Tab B.1, B.2 - EBR-003 - Calibration.xlsx]Table B.2

Print Aug-08-19 11:40:55

TABLE B.2


EAST BOULDER RIVER MODELLING ASSESSMENTMODELLED STAGE CALIBRATION STATISTICS


Page 1 of 1

B-2 of 6

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Appendix B - HEC-RAS Results\[Fig B.1 - Model Setup.xlsx]FIGURE B.1 Print 2019-08-08 11:22 AM

EAST BOULDER RIVER MODELLING ASSESSMENTHEC-RAS MODEL SETUP

FIGURE B.1


EAST BOULDER MINE

REV

P/A NO. REF. NO.

0 08AUG'19 ISSUED WITH LETTER SBF CNH

DATE DESCRIPTION PREP'D RVW'DREV

NB101-45/44 NB19-00559

0

MODEL SETUP

The model domain consisted of approximately 107,000 cells over an area of approximately 0.5 mi2. The cell discretization was set to 12 ft. in the X and Y dimensions and was refined to 4 ft. near the East Boulder River. See B.1.b.

HIGH RESOLUTION LIDAR DATA

REFINEMENT OF THE RIVER CHANNEL

B.1.b

B.1.a

B.1.c

B.1.d

Upstream Hydrograph Boundary Condition

Downstream Normal Depth Boundary Condition

CELL REFINEMENT

B-3 of 6

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Appendix B - HEC-RAS Results\[Fig B.2, B.3, B.4, Tab B.1, B.2 - EBR-003 - Calibration.xlsx]Figure B.2-Discharge Print 2019-08-08 11:43 AM

y = 0.4728x0.4401

R² = 0.7768

0

1

2

3

4

5

6

7

8

9

10

0 100 200 300 400 500 600 700 800 900

Vel

oci

ty (

ft/s

)

Discharge (cfs)

EBR-003 - Max Velocity vs. Discharge

CAL-1 n=0.065

CAL-2 n=0.055

CAL-3 n=0.06

CAL-4 n=0.04

CAL-5 n=0.08

Power (EBR-003 - Max Velocity vs. Discharge)

NOTES:1. MONITORING PERIOD FROM 2001 TO 2019 DURING APRIL TO NOVEMBER.2. MAXIMUM VELOCITY MEASUREMENTS REPRESENT MAXIMUM VALUE RECORDED AT EBR-003 WITHIN THE MEASURED PROFILE.



EBR-003 DISCHARGE MONITORING DATA AND CALIBRATION

VELOCITY VS. DISCHARGE

FIGURE B.2


EAST BOULDER MINE

REV

P/A NO. REF. NO.NB101-45/44 NB19-00559

0

B-4 of 6

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Appendix B - HEC-RAS Results\[Fig B.2, B.3, B.4, Tab B.1, B.2 - EBR-003 - Calibration.xlsx]Figure B.3-OBS vs MOD Print 2019-08-08 11:43 AM

1

2

3

4

5

6

7

8

9

10

1 2 3 4 5 6 7 8 9 10

Mo

del

ed V

elo

city

(ft

/s)

Observed Velocity (ft/s)

CAL-1 n=0.065

CAL-2 n=0.055

CAL-3 n=0.06

CAL-4 n=0.04

CAL-5 n=0.08

Match Line

NOTES:1. OBSERVED VELOCITY CALCULATED USING THE FOLLOWING REGRESSION OF THE OBSERVED DATA y = 0.4728x0.4401.



CORRELATION GRAPHOBSERVED VELOCITY VS. MODELLED VELOCITY

FIGURE B.3


EAST BOULDER MINE

REV

P/A NO. REF. NO.NB101-45/44 NB19-00559

0

B-5 of 6

I:\1\01\00045\44\A\Correspondence\NB19-00559- Letter - Detailed East Boulder River Modeling\Rev 0\Appendix B - HEC-RAS Results\[Fig B.2, B.3, B.4, Tab B.1, B.2 - EBR-003 - Calibration.xlsx]Figure B.4-WSE Print 2019-08-08 11:43 AM

y = 5E-08x3 - 5E-05x2 + 0.0168x + 6141.4R² = 0.9909

6141

6141.5

6142

6142.5

6143

6143.5

6144

0 50 100 150 200 250 300 350 400

Sta

ge

(ft)

Discharge (cfs)

STAGE DISCHARGE MEASUREMENTS AT EBR-003

CAL-7 n=0.065

CAL-8 n=0.055

CAL-9 n=0.06

CAL-10 n=0.04

CAL-11 n=0.08

Poly. (STAGE DISCHARGE MEASUREMENTS AT EBR-003)

NOTES:1. STAGE MONITORING PERIOD FROM 2015 TO 2019 DURING MARCH TO NOVEMBER.2. STAFF GAUGE WATER LEVEL IS RELATED TO LOCAL DATUM AND IS ADJUSTED USING THE LIDAR SURVEY.



EBR-003 STAGE DISCHARGE MONITORING DATAAND MODEL CALIBRATION

FIGURE B.4


EAST BOULDER MINE

REV

P/A NO. REF. NO.NB101-45/44 NB19-00559

0

B-6 of 6

August 8, 2019 NB19-00559

APPENDIX C

Photo Log

(Pages C-1 to C-5)

EAST BOULDER RIVERSITE PHOTOGRAPHS

August 8, 2019 1 of 5 NB19-00559

PHOTO 1 - Photo Area A (see Figure 1) - Monitoring Station EBR-003 stream gauge, Photo July 13, 2019, looking west.

PHOTO 2 - Photo Area A - Monitoring Station EBR-003, Photo July 13, 2019, looking west.

C-1 of 5


August 8, 2019 2 of 5 NB19-00559

PHOTO 3 - Photo Area B - Critical Section, Photo July 13, 2019, looking southwest.

PHOTO 4 - Photo Area B - Critical Section, Photo July 13, 2019, looking southeast.

C-2 of 5


August 8, 2019 3 of 5 NB19-00559

PHOTO 5 - Photo Area B - Critical Section, Photo July 13, 2019, looking southeast.

PHOTO 6 - Photo Area C - River Channel, Photo July 13, 2019, looking south.

C-3 of 5


August 8, 2019 4 of 5 NB19-00559

PHOTO 7 - Photo Area C - River Channel, Photo July 13, 2019, looking west.

PHOTO 8 - Photo Area C - Floodplain, Photo July 15, 2019, looking north.

C-4 of 5


August 8, 2019 5 of 5 NB19-00559

PHOTO 9 - Photo Area D - Monitoring Station EBR-001, Photo July 13, 2019, looking south.

PHOTO 10 - Photo Area D - Monitoring Station EBR-001, Photo July 13, 2019, looking south.

C-5 of 5

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