B08_Att_9_Floodplain_Hydrologic_Hydraulic_8-15-24.pdf

Floodplain Delineation Report

Hydrologic and Hydraulic Analysis

Replace Obsolete Housing Unit with Multiplex Unit

Devils Tower National Monument

Prepared for

Croft & Associates

3380 Blue Springs Rd

Kennesaw, GA 30144

Prepared by

EA Engineering, Science, and Technology, Inc., PBC

225 Schilling Circle, Suite 400

Hunt Valley, Maryland 21031

410-584-7000

X August 2024

Version: FINAL

EA Project No. 6353617

Page ii

EA Engineering, Science, and Technology, Inc., PBC August 2024

Devils Tower National Monument Floodplain Delineation Report

CONTENTS

Page

LIST OF TABLES ......................................................................................................................... iii LIST OF FIGURES ....................................................................................................................... iii LIST OF APPENDIXES................................................................................................................ iii LIST OF ACRONYMS AND ABBREVIATIONS ...................................................................... iv

1. Introduction

2. Hydrologic Analysis

2.1 KEYHOLE DAM DATA

2.2 USGS STATION DATA

2.3 USGS STATION ANALYSIS

2.4 HYDOLOGIC ANALYSIS RESULTS

3. Hydraulic Analysis

3.1 TOPOGRAPHY

3.2 GEOMETRY

3.3 MANNING’S N VALUES

3.4 BOUNDARY CONDITIONS AND CALCULATION SETTINGS

3.5 HYDRAULIC ANALYSIS RESULTS

4. Conclusion

REFERENCES

Page iii

LIST OF TABLES

Table 2-1. USGS Stations Reviewed

Table 2-2. USGS Station Analysis

Table 2-3. Project Site Estimated Flow Rates

Table 3-1. Project Site Water Surface Elevations

LIST OF FIGURES

Figure 2-1. USGS Station Locations

Figure 3-1. 1% and 0.2% AEP Flow Event Inundation Extents

LIST OF APPENDIXES

Appendix A: Hydrologic Supporting Documents

Appendix B: Hydraulic Supporting Documents

Appendix C: Floodplain Delineation Figures

Page iv

LIST OF ACRONYMS AND ABBREVIATIONS

AEP Annual exceedance probability cfs Cubic feet per second

EA EA Engineering, Science, and Technology, Inc., PBC

FEMA Federal Emergency Management Administration

H&H Hydrologic and Hydraulic

HEC RAS Hydrologic Engineering Center’s River Analysis System

HEC SSP Hydrologic Engineering Center’s Statistical Software Package

USGS United States Geological Survey

1. INTRODUCTION

The National Park Service requested floodplain delineations at Devils Tower National

Monument in support of task order 140P2024F0126. EA Engineering, Science, and Technology, Inc., PBC (EA) has been contracted to provide the floodplain delineations, which will be incorporated into the Floodplain Statement of Findings. EA conducted a hydrologic and hydraulic (H&H) analysis of the location identified by the client. The H&H analysis included hydrologic evaluations to determine the current 1% and 0.2% annual exceedance probability

(AEP) flood flow rates and hydraulic analysis to develop the associated floodplains for the existing conditions of the project location.

2. HYDROLOGIC ANALYSIS

A hydrologic analysis was completed for the Belle Fourche River at the project site. The hydrologic analysis was based on publicly available information. United States Geological

Survey (USGS) streamgaging stations were the primary data source for this analysis. Other relevant data sources related to the Keyhole Dam, such as the National Inventory of Dams and the Bureau of Reclamation projects and facilities data, were reviewed as well. Federal

Emergency Management Administration (FEMA) floodplain models are not available at the site.

However, a flood insurance study for a FEMA model was produced in 2014 for the town of

Hulett, Wyoming, which is approximately 10 miles downstream of the project site. Data from this study (FEMA 2007) was used to help estimate flow rates at the project site.

2.1 KEYHOLE DAM DATA

The area that drains to the project site is 2,590 square miles according to StreamStats (USGS

2024). Of that, 1,954 square miles of the project drainage area drain to and are controlled by the

Keyhole Dam. The Keyhole Dam is a federally owned, high hazard potential dam that was completed in 1952. The stated purpose of the dam is flood risk reduction and irrigation. At the time of writing this report, the Keyhole Dam does not have any outflow. The water level is at

4,092.5 feet, which is about 70% full (USBR 2024). The maximum flow of the principal outlet works is 1,480.0 cubic feet per second (cfs) when the pool is at 4,182.2 feet. Even though there is no flow and water levels are low at the time of writing this report, the structure may eventually see a rise in water levels and flow from precipitation. Federal dam safety personnel were contacted regarding the Keyhole Dam’s original design flow rates. However, limited documentation was available for the structure, and no data regarding design flows during 1% and

0.2% AEP flow events were found. The hydrologic analysis assumes that the Keyhole Dam will not undergo any major structural or operational changes that will significantly alter the design flows (i.e., the dam provides flood control and historical data is a source for estimating future

AEP discharges from the dam). The Keyhole Dam is the only major dam in the drainage area.

The Claudie Dam is a privately owned, low hazard potential dam on a tributary to the Belle

Fourche between the Keyhole Dam and the project site. Since the stated purpose of the Claudie

Dam is fire protection, stock, or small fish pond, it was assumed that the dam may not provide flood protection in the 1% or 0.2% AEP storm event. As such, no special consideration will be given to the Claudie Dam during the hydrologic analysis.

2.2 USGS STATION DATA

Ten USGS streamgaging stations were reviewed during the hydrologic analysis. Ultimately, only two stations were considered in the analysis. Table 2-1 provides a summary of station characteristics and reasons for elimination from analysis. Figure 2-1 provides a map of station locations with respect to the drainage area of the project site.

Table 2-1. USGS Stations Reviewed

Map

ID

Station

Number Station Name Period of

Record

Drainage

Area

(mi2) Included in Analysis?

1 06428200 Belle Fourche River Near

Alva, WY

9/30/1988 to

9/28/2015 2,948 No, period of record too short

2 06427500 Belle Fourche River Bel

Keyhole Reservoir, WY

4/30/1951 to

9/29/1995 1,954

Yes, contains flow from

Keyhole Dam

3 06426500 Belle Fourche River Below

Moorcroft, WY

9/30/1943 to

5/27/2024 1,690

Yes, contains flow from uncontrolled drainage area

4 06425900 Caballo C at Mouth NR

Piney WY

8/30/1977 to

9/28/1983 260 No, period of record too short

5 06425780 Belle Fourche R AB Dry C

NR Pine WY

9/30/1975 to

9/28/1983 594 No, period of record too short

6 06425720 Belle Fourche R BL

Rattlesnake C NR Piney WY

9/30/1975 to

9/29/2017

No, too far upstream of project site

7 06425750 Coal Creek NR Piney WY 9/30/1980 to

9/28/1983

71.8 No, period of record too short

8 06426400 Donkey C NR Moorcroft

WY

8/30/1977 to

10/7/1981 246 No, period of record too short

9 06426160 Stonepile Creek at Mouth, Near Gilette WY

7/4/2000 to

10/29/2016 14.5

No, drainage area contains too much developed area

10 06426130 Donkey Creek Near Gillette

WY

7/4/2000 to

11/6/2016 63.4

No, drainage area contains too much developed area

Notes:

mi2 = square miles

Figure 2-1. USGS Station Locations

Blue Triangle = Flow Gaging Station

Black Triangles = Undefined Station Type

Yellow Area = Project Drainage Area

Red Circle = Project Area

2.3 USGS STATION ANALYSIS

The two stations identified in Table 2-1 were analyzed using the Hydrologic Engineering

Center’s Statistical Software Package (HEC SSP). Within HEC SSP the two stations were analyzed using different methods. The station below the Keyhole Reservoir (Keyhole Station) was analyzed using a graphical frequency analysis due to the impact of the Keyhole Dam on water flow. The station below Moorcroft (Moorcroft Station) was analyzed using a Bulletin 17C analysis, which is applicable to stations not significantly altered by flood control reservoirs.

These two methods were used to determine the 1% and 0.2% AEP flow rates at each station. The results of the HEC SSP analysis are shown in Table 2-2.

Table 2-2. USGS Station Analysis

Map

ID

Station

Number Station Name

Short Station

Name

Drainage

Area (mi2)

1% AEP

Flow (cfs)

0.2 % AEP

Flow (cfs)

2 06427500 Belle Fourche River Bel

Keyhole Reservoir, WY Keyhole 1,954 1,970 4,095

3 06426500 Belle Fourche River Below

Moorcroft, WY Moorcroft 1,690 7,734 13,595 mi2 = square miles

Neither of the two stations are within the immediate vicinity of the project area, so flows in

Table 2-2 were scaled to obtain an estimate of peak flows at the project site. The Keyhole Station is immediately downstream of the Keyhole Reservoir, so the 1% and 0.2% AEP flow rates at this station were directly used to estimate flow from all drainage area upstream of the dam. The

Moorcroft Station flow rates represent uncontrolled flow from a drainage area with similar topography to the uncontrolled area between the dam and project site. Therefore, the 1% and

0.2% AEP flow rates from the Moorcroft Station were linearly scaled to the uncontrolled area

(636 square miles) between the dam and project site using the equation below. The scaled flow was added to the Keyhole Station flow rates to estimate the project site flow rates.

𝑆𝑐𝑎𝑙𝑒𝑑 𝐹𝑙𝑜𝑤 = 𝑀𝑜𝑜𝑟𝑐𝑟𝑜𝑓𝑡 𝑆𝑡𝑎𝑡𝑖𝑜𝑛 𝐹𝑙𝑜𝑤

𝑀𝑜𝑜𝑟𝑐𝑟𝑜𝑓𝑡 𝑆𝑡𝑎𝑡𝑖𝑜𝑛 𝐴𝑟𝑒𝑎 ∗ 𝑈𝑛𝑐𝑜𝑛𝑡𝑟𝑜𝑙𝑙𝑒𝑑 𝐴𝑟𝑒𝑎

2.4 HYDOLOGIC ANALYSIS RESULTS

Several approaches for estimating the project site flow rates were considered for this analysis.

The final methodology was selected because it used data from the nearest streamgaging stations with the most years of peak flow data (Keyhole Station with 43 years and Moorcroft Station with

71 years). The results are within expected values when compared to other methodologies that were considered. Also, the estimated flow rates are below those estimated by FEMA for the town of Hulett, which is to be expected since Hulett is downstream of the project site. The results from the hydrologic analysis are provided in Table 2-3.

Table 2-3. Project Site Estimated Flow Rates

1% AEP (cfs) 0.2% AEP (cfs)

4,881 9,212

3. HYDRAULIC ANALYSIS

A hydraulic analysis was completed for the Belle Fourche River at the project site. The hydraulic analysis was based on publicly available information. The results of the hydrologic analysis, USGS LiDAR, aerial imagery, and photos from previous site visits were the main data sources.

The hydraulic analysis was completed using Hydrologic Engineering Center’s River Analysis

System (HEC-RAS) 1D Steady Flow Analysis. Settings and data inputs used in HEC-RAS are described in the following sections.

3.1 TOPOGRAPHY

Topographic data were based on published 1-meter digital elevation model (DEM) elevation products (i.e., LiDAR) from the National Map (USGS 2022) and were accessed through the RAS

Mapper terrain downloader. This LiDAR product is prefiltered to minimize the inclusion of vegetation and structures as ground points. Since the project area is not densely vegetated or developed, the filtering should be sufficient to use the LiDAR without additional manipulation to remove vegetation or structures. One potential shortcoming of LiDAR is that it does not penetrate water. This means that the channel topography below the water level at the time of the

LiDAR flight is not captured on the surface. Since the modeled events are much larger than average flow, the lost channel capacity is unlikely to have any meaningful impact on the result of the model. As a result, no corrections were made to reinforce a deeper thalweg into the modeled channel.

3.2 GEOMETRY

EA used RAS Mapper to extract cross sections from the topographic data. Cross sections were set perpendicular to flow, located immediately upstream and downstream of structures, and capture the entire width of the floodplains. The bridge south of the project area on Devils Tower

National Monument Road was included in the model because it could impact the hydraulic results. Structure data were estimated based on photos and imagery taken in the field. Flow paths were used to update the right and left overbank reach lengths from the default centerline reach length.

3.3 MANNING’S N VALUES

EA used visual observations and the Cowan’s method to calculate an average Manning’s n value for the channel and overbanks of the modeled area. See Appendix B for supporting documents.

N values used in the model were adjusted from the calculated n values when conditions at a cross section were rougher or smoother than the average. Channel n values were 0.048. An average overbank n value of 0.052 was calculated. Overbank n values ranged from 0.049 to 0.056 in the

HEC RAS model due to localized variation in overbank conditions.

3.4 BOUNDARY CONDITIONS AND CALCULATION SETTINGS

The site was modeled using the steady state flow analysis method. The model used the subcritical flow calculation set. Peak flow values were selected as described in Sections 2.

Normal depth was selected for the downstream boundary condition. Normal depth was calculated for the model using the slope at the cross section at the downstream limits of the model.

3.5 HYDRAULIC ANALYSIS RESULTS

The model generated inundation boundaries and water surface elevations at the project site for the 1% and 0.2% AEP flow events. Water surface elevations at the project site during 1% and

0.2% AEP flow events are provided in Table 3-1.

Table 3-1. Project Site Water Surface Elevations

River Section/Area 1% AEP Elevation (feet) 0.2% AEP Elevation (feet)

Section 14162 3,864.47 3,866.60

Section 12024 3,862.76 3,864.64

Near Proposed Building 3,863.6 3,865.7

Figure 3-1 shows the inundation extents for the flow events near the project site. More detailed results and figures are available in Appendices B and C. GIS shapefiles of the inundation boundaries will also be transmitted with this report.

Figure 3-1. 1% and 0.2% AEP Flow Event Inundation Extents

4. CONCLUSION

Portions of the project area are within the 1% and 0.2% AEP flow event inundation. The

Floodplain Statement of Findings will be revised to incorporate the results of this floodplain delineation.

REFERENCES

Federal Emergency Management Agency. 2006. Flood Insurance Study: Town of Hulett, Wyoming Crook County.

United States Bureau of Reclamation. 2024. Missouri Basin and Arkansas-Rio Grande-Texas

Gulf Regions: Current Reservoir Data for Keyhole Reservoir, WY.

United States Geological Survey. 2024. StreamStats. https://www.usgs.gov/streamstats

———. 2022. National Map Data Download Application.

https://www.usgs.gov/tools/download-data-maps-national-map https://www.usgs.gov/streamstats https://www.usgs.gov/tools/download-data-maps-national-map

Appendix A

Appendix A:

Hydrologic Supporting Documents

Project: Project No.

Sheet No. 1 of 1

Subject: Dwg No. XX

Computed by: RVB/ML Date: 8/8/24 Checked by: LLR Date: 8/13/24

Objective

Determine the 1% AEP and 0.2% AEP flow events at Devil's Tower National Monument

Data

Source Notes

Map ID

Station

Number

Drainage

Area (mi^2)

Avg

Daily

Flow

1 06428200 2948 83.896

2 06427500 1954 22

3 06426500 1690 23.42

4 06425900 260 2.57

5 06425780 594 4.36

6 06425720 495 2.495

7 06425750 71.8 1.1

8 06426400 246 10.15

9 06426160 14.5 3.8

10 06426130 63.4 1.33

On main river (Belle Fourche)

2590 n/a

DETO Housing 6353605

USGS Station Review and Analysis

USGS Stream Gauges

Station Name Period of Record

Upstream/Downstream of Dam In/Out of Watershed Link

Belle Fourche River Bel

Keyhole Reservoir, WY 4/30/1951-9/29/1995 Downstream Inside https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedi

Belle Fourche River Near

Alva, WY 9/30/1988-9/28/2015 Downstream Outside https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedi

Caballo C at Mouth NR Piney

WY 8/30/1977-9/28/1983 Upstream Inside https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true

Belle Fourche River Below

Moorcroft, WY 9/30/1943-5/27/2024 Upstream Inside https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedi

Belle Fourche R BL

Rattlesnake C NR Piney WY 9/30/1975-9/29/2017 Upstream Inside https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedi

Belle Fourche R AB Dry C NR

Pine WY 9/30/1975 - 9/28/1983 Upstream Inside https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true

Donkey C NR Moorcroft WY 8/30/1977-10/7/1981 Upstream Inside https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true

Coal Creek NR Piney WY 9/30/1980-9/28/1983 Upstream Inside https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true

Donkey Creek Near Gillette

WY 7/4/2000 - 11/6/2016 Upstream Inside https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedi

Stonepile Creek at Mouth, Near Gilette WY 7/4/2000 - 10/29/2016 Upstream Inside https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedi

Whole Basin

Belle Fourche River Near

Devils Tower n/a n/an/a https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedian=false https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedian=false https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedian=false https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedian=false https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedian=false https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedian=false https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06427500/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06428200/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425900/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedian=false https://waterdata.usgs.gov/monitoring-location/06426500/#parameterCode=00065&period=P7D&showMedian=false https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425720/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425780/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426400/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06425750/#period=P1Y&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426130/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedian=true https://waterdata.usgs.gov/monitoring-location/06426160/#parameterCode=00060&period=P7D&showMedian=true

Procedure

HEC SSP Results

Map ID

Station

Number

Drainage

Area (mi^2)

1% AEP

peak flow

(cfs)

0.2% AEP

peak flow

(cfs)

1% AEP

flow/mi^2

0.2% AEP

flow/mi^2

2 06427500 1954 1970 4095 1.01 2.10

3 06426500 1690 7734 13595 4.58 8.04

Scaled Peak Flows

Whole Basin 2590 mi^2

Area between Dam and Project Site 636 mi^2

Basin Flow Based on Keyhole Dam Gage (USGS 06427500) and directly scaled USGS gage below Moorcroft (USGS 06426500) to the area between the Keyhole Reservoir and the project site.

06427500 plus scaled 06426500

4881 cfs

9212 cfs

Conclusion

The basin flows from the paired graphical frequency and Bulletin 17C analyses using the USGS gaging station below the Keyhole Dam plus a scaled flowrate from the station near Moorcroft were selected for use in the hydraulic model. These flows was selected due their direct incorporation of dam flows and utilizing a gage with a long period of record and similar drainage area to provide an estimate flow value for the uncontrolled area between the dam and project site.

1% AEP flow (cfs)

0.2% AEP flow (cfs)

Stations were analyzed in HEC SSP to determine the 1% and 0.2% AEP flows at each station. Stations 9 & 10 were eliminated prior to HEC SSP analysis due to the impact of the city of Gillette on their drainage areas. Stations 1, 4, 5, & 8 were eliminated by HEC SSP for their short periods of record. Station 2 was analyzed using a graphical frequency analysis (significant control from Keyhole Dam) and staion 3 was analyzed using a Bulletin 17C analysis.

Station 2 provides direct data from the keyhole dam, and station 3 provides an uncontrolled flow rate that can be scaled to the uncontrolled area between the dam and project site.

Station Name

Belle Fourche River Bel Belle Fourche River Below

Moorcroft, WY

Appendix B

Appendix B:

Hydraulic Supporting Documents

Sheet No. 1 of 3

Subject: Dwg No. XX

Computed by: RVB Date: 6/6/24 Checked by: LLR Date: 8/13/24 Objective Estimate Manning's n value of channel bed and overbanks for existing conditions.

Procedure Manning's n values are determined from the table below.

DETO Housing 6353605

Existing Conditions Manning's n

Sheet No. 2 of 3

Subject: Dwg No. XX

Computed by: RVB Date: 6/6/24 Checked by: LLR Date: 8/13/24

Existing Conditions Manning's n

DETO Housing 6353605

These Cowan's Method tables are taken from the Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains , USGS 1989

Sheet No. 3 of 3

Subject: Dwg No. XX

Computed by: RVB Date: 6/6/24 Checked by: LLR Date: 8/13/24 Objective Estimate Manning's n value of channel bed and overbanks for existing conditions.

Procedure

Cowan's Method Equation

Channel n Notes nb = 0.022 Sandy Loam - Web Soil Survey n1 = 0.006 Moderately eroded side slopes n2 = 0.003 Alternating Ocassionally, some narrowing and widening, but not significant amounts n3 = 0.002 Negligible - some rocks n4 = 0.004 Very little vegetation in channel L1 = 23340 ft Channel Length - from RAS centerline L2 = 12653 ft Valley Length - estimated on google earth

Ratio = 1.84 m = 1.30 Severe n = 0.0481 for channel bed

Flood-Plain n Notes nb = 0.029 Firm Soil n1 = 0.008 Moderate, rises and dips due to oxbows n2 = 0 NA n3 = 0.005 Obstructions occupy less that 15% of cross sectional area n4 = 0.010 Small, short thinly vegetated pasture, scattereed trees accounted for in n3 m = 1.00 NA n = 0.052

Grassy pasture, some medium height grass

Conclusion n Channel Bed 0.048 Channel Left and Right overbanks 0.052 Overbank

For left and right overbanks

DETO Housing 6353605

Sample Conditions Manning's n

Use Cowan's Method described in the Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains, USGS 1989

Note: Use tables provided in Cowan's Method for selecting Manning's n values. Use Table 1 for Base values of Manning's n, Table 2 for channel bed n values, and Table 3 for left and right overbank n values. Tables are shown on Sheet 2.

0 5000 10000 15000 20000 25000

DETO Plan: DETO Existing Conditions 8/12/2024

Main Channel Distance (ft)

El ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

WS InBank capacity

Ground

Belle Fourche Reach 1

0 500 1000 1500 2000 2500 3000 3500

DETO Plan: DETO Existing Conditions 8/12/2024

Station (ft)

El ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.049 .

.049 ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.049 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.049 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.049 .

0 500 1000 1500 2000 2500 3000 3500 4000 ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.049 .

0 500 1000 1500 2000 2500 3000 ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.052 .

.052 ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.054 .048 .054

0 500 1000 1500 2000 2500 ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.054 .

.054 ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.054 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.054 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.054 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.052 .

0 500 1000 1500 2000 2500 ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.052 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.052 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.052 .

DETO Plan: DETO Existing Conditions 8/12/2024 Highway 110/ CR 174

Station (ft)

El ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.052 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.052 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.054 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Ineff

Bank Sta

.054 .

ev at io n (ft

Legend

WS 0.2% AEP

WS 1% AEP

Ground

Bank Sta

.054 .

HEC-RAS Plan: Existing_UpdatedFlow River: Belle Fourche Reach: Reach 1 Reach River Sta Profile Q Total Min Ch El W.S. Elev Crit W.S. E.G. Elev E.G. Slope Vel Chnl Flow Area Top Width Froude # Chl

(cfs) (ft) (ft) (ft) (ft) (ft/ft) (ft/s) (sq ft) (ft) Reach 1 23990 1% AEP 4881.00 3863.63 3874.08 3869.48 3874.32 0.001184 4.31 2110.97 2116.30 0.26 Reach 1 23990 0.2% AEP 9212.00 3863.63 3875.64 3875.74 0.000654 3.60 5244.92 2394.72 0.20

Reach 1 19544 1% AEP 4881.00 3858.39 3868.35 3868.68 0.001508 4.93 1117.32 217.90 0.30 Reach 1 19544 0.2% AEP 9212.00 3858.39 3870.17 3870.96 0.002767 7.62 1486.07 536.94 0.42

Reach 1 18578 1% AEP 4881.00 3857.69 3867.85 3867.90 0.000401 2.53 3253.57 1382.71 0.15 Reach 1 18578 0.2% AEP 9212.00 3857.69 3869.80 3869.86 0.000422 2.99 5863.20 2018.33 0.16

Reach 1 16619 1% AEP 4881.00 3856.75 3867.02 3867.15 0.000606 3.08 2107.44 664.48 0.19 Reach 1 16619 0.2% AEP 9212.00 3856.75 3868.98 3869.14 0.000653 3.69 3500.01 1262.76 0.20

Reach 1 15640 1% AEP 4881.00 3856.47 3866.15 3866.35 0.001177 3.79 1597.35 1048.20 0.26 Reach 1 15640 0.2% AEP 9212.00 3856.47 3868.07 3868.31 0.001196 4.51 2844.90 1682.77 0.27

Reach 1 14162 1% AEP 4881.00 3855.91 3864.47 3864.61 0.001197 3.48 1904.96 581.15 0.25 Reach 1 14162 0.2% AEP 9212.00 3855.91 3866.60 3866.76 0.000952 3.81 3171.83 991.92 0.24

Reach 1 13212 1% AEP 4881.00 3854.94 3863.61 3863.75 0.000784 3.20 1835.77 535.83 0.21 Reach 1 13212 0.2% AEP 9212.00 3854.94 3865.69 3865.90 0.000969 4.22 2903.70 1346.87 0.25

Reach 1 12024 1% AEP 4881.00 3852.63 3862.76 3862.89 0.000795 3.44 1990.50 556.24 0.22 Reach 1 12024 0.2% AEP 9212.00 3852.63 3864.64 3864.85 0.001019 4.49 2888.44 1720.50 0.25

Reach 1 11211 1% AEP 4881.00 3852.50 3862.14 3862.26 0.000822 3.64 2303.00 804.67 0.22 Reach 1 11211 0.2% AEP 9212.00 3852.50 3863.61 3863.87 0.001614 5.68 3358.28 1470.07 0.32

Reach 1 10407 1% AEP 4881.00 3852.79 3861.77 3861.81 0.000343 2.39 3930.85 1249.12 0.14 Reach 1 10407 0.2% AEP 9212.00 3852.79 3863.14 3863.19 0.000411 2.89 5680.67 1285.47 0.16

Reach 1 8402 1% AEP 4881.00 3850.67 3860.19 3856.37 3860.55 0.002277 5.04 1205.51 1565.37 0.35 Reach 1 8402 0.2% AEP 9212.00 3850.67 3861.96 3862.16 0.001372 4.60 3791.87 1604.12 0.28

Reach 1 5938 1% AEP 4881.00 3848.68 3856.93 3857.04 0.001040 3.63 2010.62 943.92 0.24 Reach 1 5938 0.2% AEP 9212.00 3848.68 3859.51 3859.65 0.001099 4.60 4160.25 1408.75 0.26

Reach 1 5274 1% AEP 4881.00 3846.12 3856.13 3856.37 0.001100 4.34 1359.84 557.42 0.26 Reach 1 5274 0.2% AEP 9212.00 3846.12 3858.82 3859.03 0.000899 4.70 3519.46 1036.39 0.24

Reach 1 4337 1% AEP 4881.00 3844.94 3854.94 3855.22 0.001347 4.19 1168.84 727.46 0.27 Reach 1 4337 0.2% AEP 9212.00 3844.94 3857.50 3857.92 0.001511 5.43 2026.48 1396.42 0.31

Reach 1 3406 1% AEP 4881.00 3843.77 3854.49 3848.80 3854.56 0.000337 2.61 2339.24 570.96 0.15 Reach 1 3406 0.2% AEP 9212.00 3843.77 3856.92 3849.95 3857.07 0.000475 3.59 3119.08 730.27 0.18

Reach 1 3331 DETO Road Bridge

Reach 1 3205 1% AEP 4881.00 3842.85 3854.37 3854.47 0.000433 3.01 2140.57 647.71 0.17 Reach 1 3205 0.2% AEP 9212.00 3842.85 3856.75 3856.93 0.000586 4.03 2904.74 760.39 0.20

Reach 1 2274 1% AEP 4881.00 3841.99 3853.92 3854.03 0.000580 3.55 2587.95 885.12 0.19 Reach 1 2274 0.2% AEP 9212.00 3841.99 3856.32 3856.42 0.000504 3.79 4830.14 1066.28 0.19

Reach 1 1439 1% AEP 4881.00 3841.70 3852.80 3853.21 0.001970 5.19 939.92 865.61 0.33 Reach 1 1439 0.2% AEP 9212.00 3841.70 3854.69 3850.42 3855.53 0.003202 7.48 1576.59 999.06 0.44

Reach 1 650 1% AEP 4881.00 3840.88 3851.28 3847.24 3851.63 0.002001 5.03 1314.21 529.42 0.33 Reach 1 650 0.2% AEP 9212.00 3840.88 3853.04 3851.03 3853.42 0.002003 5.83 2345.72 608.88 0.35

Appendix C

Appendix C:

Floodplain Delineation Figures

15640

18578

16619

19544

23990

14162

13212

1121112024

Legend

Approximate Project Area

Cross Sections

Centerline

0.2% AEP Inundation

1% AEP Inundation

Figure 1 Full Model Extents

Flood Inundation Boundaries Devils Tower National Monument, WY

Map Date: 8/13/2024

0 750 1,500

Feet $

14162

13212

12024

Legend

Approximate Project Area

Cross Sections

Centerline

0.2% AEP Inundation

1% AEP Inundation

Figure 2 Project Area

Flood Inundation Boundaries Devils Tower National Monument, WY

Map Date: 8/13/2024

0 200 400

Feet $