SWAIN-010 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)

CHEOAH VALLEY DAM

SWAIN-010

SWAIN COUNTY, NC

PREPARED BY:
WOOD E&I Solutions

2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Cheoah Valley Dam in Swain County, North
Carolina. The purpose of the study was to analyze the existing potential of the dam for
overtopping during predetermined specified rainfall events. The analyses were completed using
the United States Army Corps of Engineers developed software Hydrologic Engineering Center –
Hydrologic Modeling System (HEC-HMS) in combination with Arc-GIS to develop required input
parameters. The rainfall events included in the analyses consist of 24-hour durations with return
periods of 25 years, 50 years, 100 years, 200 years, 500 years, and 1000 years rainfall events.
Results of the analyses indicate that this structure is hydraulically adequate for the 25 yea rs, 50
years, 100 years, 200 years, 500 years, and 1000 years rainfall events. Data, input parameters,
and final results can be found on the attached summary sheets and in the appendices of this
report as well as in the associated electronic file in which all program files are stored.
Drainage Area (Sq. Mi.)=
Curve Number =
Lag Time (min.)=
Rainfall Distribution Methods =
Concrete-Lined Rectangular Channel
NOT TO SCALE
157.90
168.70
179.80
191.10
202.70
207.40
1637.30
1637.80
1638.00
STORAGE (ACRE-FT)
98.10
107.50
117.00
126.80
136.90
147.30
1634.30
1634.80
1635.30
1635.80
1636.30
1636.80
45.50
53.80
62.30
71.00
79.80
88.90
1631.30
1631.80
1632.30
STORAGE (ACRE-FT)
0.00
7.10
14.40
21.80
29.50
37.40
1628.30
1628.80
1629.30
1629.80
1630.30
1630.80
1626.8
N/A
ELEVATION
1626.80
1627.30
1627.80
ELEVATION
1632.80
1633.30
1633.80
47.8
33
SCS Type 2
SPILLWAY
Principal
Auxiliary
TYPE
None
ELEVATION
24 HOUR - 200 YEAR EVENT, 8.26 INCHES, ELEVATION 1628.9 (3.4 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 7.49 INCHES, ELEVATION 1628.5 (3.8 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 6.75 INCHES, ELEVATION 1628.1 (4.2 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.02 INCHES, ELEVATION 1627.8 (4.5 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.7057
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
CHEOAH VALLEY DAM, SWAIN-010
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
TOP OF DAM ELEVATION = 1632.3
24 HOUR - 1000 YEAR EVENT, 10.2 INCHES, ELEVATION 1630 (2.3 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 9.33 INCHES, ELEVATION 1629.5 (2.8 FT FREE-BOARD)
CHEOAH VALLEY DAM SWAIN-010 DECEMBER 23, 2020

APPENDIX A
Watershed Boundary

Sources: Esri, HERE, Garmin, USGS, Intermap, INCREMENT P,
NRCan, Esri Japan, METI, Esri China (Hong Kong), Esri Korea,
Esri (Thailand), NGCC, (c) OpenStreetMap contributors, and the
GIS User Community
Cheoh Valley Dam - SWAIN-010 - Watershed Boundary
Legend
LongestFlowPath
Watershed Boundary
0 0.25 0.50.125 Miles
Date: 12/16/2020
/Drainage Area: 0.7057 Square Miles
Flowpath Length: 1.3215 Miles

APPENDIX B
Lag Time Calculation

Dam No: SWAIN-010 Date: 12/12/2020
Dam Name: Cheoah Valley Dam Engineer: E.Okorie
Maximum Potential Retention (inches):S = (1000/cn')-10
Time of Concentration (hours):Tc =l0.8*(S+1)0.7
1140*Y0.5
Basin Data:Input Values in yellow
Curve Number CN = 47.80 from NRCS CN
S =10.9205 in.
Mean Basin Slope Y = 42.06 % from Watershed Mean slope
Longest Flowpath: 1.3215 mi. from Watershed Longest flowpath
Flow Length l =6977.52 ft.
Time of Concentration Tc =0.91 Hours Tc =55 mins
Lag Time TL =0.55 Hours TL =33 mins

APPENDIX C
NRCS Curve Number Summary Sheet

Change CN according to TR-55 (see tables below)
NLCD (2016) Landuse TR-55 Landuse HSG A (sq. ft.)CN HSG B (sq. ft.)CN HSG C (sq. ft.)CN HSG D (sq. ft.)CN
Open Water Not Available 15274 100 1527424 352761 100 35276128 0 100 0 0 100 0
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)189432 49 9282155 147426 69 10172428 0 79 0 0 84 0
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 0 72 0 0 81 0 0 86 0
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 0 85 0 0 90 0 0 92 0
Developed, High Intensity Urban districts - Commercial and business 0 89 0 0 92 0 0 94 0 0 95 0
Barren Land Newly graded areas 0 77 0 0 86 0 0 91 0 0 94 0
Deciduous Forest Woods - Fair 5493265 36 197757554 2436734 60 146204015 0 73 0 724939 79 57270176
Evergreen Forest Woods - Fair 795447 36 28636087 571550 60 34293004 0 73 0 58111 79 4590769
Mixed Forest Woods - Fair 5254273 36 189153828 3125114 60 187506819 0 73 0 452648 79 35759209
Shrub/Scrub Brush - Fair 7230 35 253034 50881 56 2849353 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 0 71 0 0 81 0 0 89 0
Hay/Pasture Pasture - Fair 0 49 0 0 69 0 0 79 0 0 84 0
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 0 80 0 0 87 0 0 90 0
Woody Wetlands Not Available 0 80 0 0 85 0 0 90 0 0 95 0
Emergent Herbaceuous Wetlands Not Available 0 80 0 0 85 0 0 90 0 0 95 0 Weighted CN=47.80
NOTE: Curve Numbers are from SCS TR-55 with the exception of assumed HSG A (CN=60) for Herbaceous, Open Water, Woody Wetlands, and Emergent Herbaceous Wetlands.
NOTE: double-check all the land types are being calculated and texts are accurate
NOTE: double-check the exported attribute table has the same table format as the "Export_Output" table

APPENDIX D

Hyetograph Results Used for Analyses
0.0
2.0
4.0
6.0
8.0
10.0
12.0
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24Cumulative Depth (in)Time (hrs)
Hyetographs for 24-Hour SCS Type II Rainfall Distributions
25YR
50YR
100YR
200YR
500YR
1000YR

APPENDIX E
NOAA Atlas 14 Rainfall Table

Point precipitation frequency estimates (inches)
NOAA Atlas 14 Volume 2 Version 3
Data type: Precipitation depth
Time series type: Partial duration
Project area: Ohio River Basin
Location name (ESRI Maps): Robbinsville, North Carolina USA
Station Name: -
Latitude: 35.4663°
Longitude: -83.9322°
Elevation (USGS): 1777.5 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.354 0.41 0.473 0.551 0.635 0.712 0.784 0.856 0.947 1.04
10-min:0.565 0.655 0.758 0.882 1.01 1.13 1.25 1.36 1.5 1.64
15-min:0.707 0.824 0.959 1.12 1.28 1.44 1.57 1.71 1.89 2.05
30-min:0.969 1.14 1.36 1.62 1.9 2.16 2.41 2.67 3 3.32
60-min:1.21 1.43 1.75 2.1 2.53 2.93 3.32 3.74 4.3 4.85
2-hr:1.43 1.68 2.05 2.47 2.96 3.44 3.91 4.42 5.1 5.77
3-hr:1.56 1.83 2.21 2.65 3.19 3.72 4.24 4.81 5.6 6.37
6-hr:1.98 2.3 2.73 3.25 3.89 4.52 5.16 5.85 6.81 7.75
12-hr:2.55 2.96 3.5 4.12 4.86 5.55 6.23 6.94 7.87 8.76
24-hr:3.06 3.66 4.47 5.12 6.02 6.75 7.49 8.26 9.33 10.2
2-day:3.7 4.43 5.4 6.18 7.23 8.08 8.93 9.82 11 12
3-day:4.01 4.79 5.82 6.62 7.7 8.54 9.4 10.3 11.5 12.4
4-day:4.32 5.16 6.24 7.06 8.16 9.01 9.87 10.7 11.9 12.8
7-day:5.28 6.3 7.55 8.5 9.75 10.7 11.6 12.6 13.8 14.8
10-day:6.08 7.21 8.57 9.62 11 12.1 13.1 14.2 15.5 16.6
20-day:8.37 9.88 11.5 12.6 14.1 15.1 16.1 17.1 18.2 19.1
30-day:10.2 12 13.7 15 16.5 17.6 18.6 19.5 20.6 21.4
45-day:12.9 15.1 17.1 18.6 20.3 21.5 22.7 23.7 24.9 25.7
60-day:15.6 18.2 20.5 22.2 24.1 25.5 26.8 27.9 29.2 30.1
Date/time (GMT): Sun Dec 13 03:50:33 2020
pyRunTime: 0.0265591144562

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client: NCEM Date: 12/12/2020
Project: Overtopping Study Engineer: E.Okorie
Dam: Cheoah Valley Dam Dam No. SWAIN-010
Reservior Water
Level (RWS)
Elevation (ft)
Primary 1 spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
1626.8 0.0 0.0
1627.0 3.9 3.9
1627.5 25.6 25.6
1628.0 57.5 57.5
1628.5 96.9 96.9
1629.0 142.6 142.6
1629.5 193.9 193.9
1630.0 250.2 250.2
1630.5 311.1 311.1
1631.0 376.2 376.2
1631.5 445.4 445.4
1632.0 518.3 518.3
1632.5 594.8 594.8
1633.0 674.8 674.8
1633.5 758.0 758.0
1634.0 844.5 844.5
1634.5 933.9 933.9
1635.0 1026.4 1026.4
1635.5 1121.7 1121.7
1636.0 1219.7 1219.7
1636.5 1320.5 1320.5
1637.0 1423.9 1423.9
1637.5 1529.9 1529.9
1638.0 1638.4 1638.4
1638.5 1749.3 1749.3
1639.0 1862.6 1862.6
1639.5 1978.3 1978.3
1640.0 2096.2 2096.2
0.0
500.0
1000.0
1500.0
2000.0
2500.0
1626.0 1628.0 1630.0 1632.0 1634.0 1636.0 1638.0 1640.0 1642.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)