SWAIN-003 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)

BRYSON CITY WATER SUPPLY DAM

SWAIN-003

SWAIN COUNTY, NC

PREPARED BY:
WOOD E&I Solutions

2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Bryson City Water Supply 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. Results of the analyses indicate that this structure is hydraulically adequate for the 25 years
storm events. The structure is over-topped by the 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 =
20' Riser Tower w/ 6.9' x 6.2' Box culvert
SUMMARY OF INPUT PARAMETERS
2.668
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
BRYSON CITY WATER SUPPLY DAM, SWAIN-003
ANALYSIS PERFORMED BY: E.OKORIE, WOOD
24 HOUR - 100 YEAR EVENT, 8.04 INCHES, ELEVATION 2331 (1.2 FT OVER-TOPPED)
24 HOUR - 50 YEAR EVENT, 7.17 INCHES, ELEVATION 2330.3 (0.5 FT OVER-TOPPED)
TOP OF DAM ELEVATION = 2329.8
24 HOUR - 25 YEAR EVENT, 6.35 INCHES, ELEVATION 2324.9 (4.9 FT FREE-BOARD)
24 HOUR - 1000 YEAR EVENT, 11.4 INCHES, ELEVATION 2332.4 (2.6 FT OVER-TOPPED)
24 HOUR - 500 YEAR EVENT, 10.3 INCHES, ELEVATION 2332 (2.2 FT OVER-TOPPED)
24 HOUR - 200 YEAR EVENT, 8.96 INCHES, ELEVATION 2331.5 (1.7 FT OVER-TOPPED)
2306.6
2317.6
58.5
42
SCS Type 2
SPILLWAY
Principal
Auxiliary
TYPE
24" Riser w/ 12-inch DIP Barrel
ELEVATION
51.603.102311.10 2328.10
ELEVATION
2323.60
2324.10
2324.60
2325.10
2325.60
34.60
36.40
38.20
40.00
41.90
43.80
45.70
47.70
49.60
2326.10
2326.60
2327.10
2327.60
2309.60
2310.10
2310.60
2.10
STORAGE (ACRE-FT)ELEVATION
2306.60
2307.10
2307.60
STORAGE (ACRE-FT)
0.00
0.30
0.70
1.00
1.30
1.70
2308.10
2308.60
2309.10
2.40
2.80
BRYSON CITY WATER SUPPLY DAM SWAIN-003 DECEMBER 23, 2020
2315.10
2315.60
2316.10
2316.60
2311.60
2312.10
2312.60
2313.10
2313.60
81.70
84.30
2317.60
2318.10
71.70
74.10
76.60
79.20
57.90
60.00
62.20
64.50
66.80
69.20
53.70
3.90
4.30
4.80
2317.10
2314.10
2314.60
111.00
NOT TO SCALE
86.90
89.50
92.20
94.90
97.60
100.30
2323.10
2320.10
2320.60
2321.10
2321.60
2322.10
2322.60
2318.60
2319.10
2319.60
103.10
55.80
2339.10
2339.60
2340.00
2336.10
2336.60
2337.10
2337.60
2338.10
2338.60
2333.10
2333.60
2334.10
2334.60
2335.10
2335.60
2330.10
2330.60
2331.10
2331.60
2332.10
2332.60
105.90
108.70
2328.60
2329.10
2329.60
31.10
32.80
21.50
23.00
24.50
26.10
27.80
29.40
12.90
14.20
15.60
17.00
18.50
19.90
5.60
6.60
7.80
9.00
10.30
11.60
3.50
BRYSON CITY WATER SUPPLY DAM SWAIN-003 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
Bryson City WS Dam - SWAIN-003 - Watershed Boundary
Legend
LongestFlowPath
Watershed Boundary
0 0.5 10.25 Miles
Date: 12/16/2020
/Drainage Area: 2.668 Square Miles
Flowpath Length: 3.093 Miles

APPENDIX B
Lag Time Calculation

Dam No: SWAIN-003 Date: 12/9/2020
Dam Name: Bryson City Water Supply 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 = 58.50 from NRCS CN
S =7.09402 in.
Mean Basin Slope Y = 57.17 % from Watershed Mean slope
Longest Flowpath: 3.09288 mi. from Watershed Longest flowpath
Flow Length l =16330.4 ft.
Time of Concentration Tc =1.18 Hours Tc =71 mins
Lag Time TL =0.71 Hours TL =42 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 0 100 0 0 100 0 0 100 0 0 100 0
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)11302 49 553805 123475 69 8519791 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 7674874 36 276295452 43034139 60 2582048358 60123 73 4388980 3707136 79 292863767
Evergreen Forest Woods - Fair 230578 36 8300810 597047 60 35822834 0 73 0 41741 79 3297541
Mixed Forest Woods - Fair 1574786 36 56692284 14805721 60 888343287 0 73 0 2316110 79 182972696
Shrub/Scrub Brush - Fair 0 35 0 193704 56 10847400 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=58.50
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): Bryson City, Nortch Carolina USA
Station Name: -
Latitude: 35.4820°
Longitude: -83.4679°
Elevation (USGS): 2905.98 ft
PRECIPITATION FREQUENCY ESTIMATES
by duration for
ARI (years):1 2 5 10 25 50 100 200 500 1000
5-min:0.35 0.41 0.48 0.552 0.634 0.704 0.772 0.841 0.927 1.01
10-min:0.558 0.655 0.769 0.883 1.01 1.12 1.23 1.33 1.47 1.59
15-min:0.698 0.824 0.973 1.12 1.28 1.42 1.55 1.68 1.85 1.99
30-min:0.957 1.14 1.38 1.62 1.9 2.14 2.38 2.62 2.94 3.22
60-min:1.19 1.43 1.77 2.11 2.53 2.9 3.27 3.67 4.21 4.71
2-hr:1.4 1.68 2.07 2.46 2.95 3.39 3.83 4.31 4.95 5.55
3-hr:1.54 1.83 2.24 2.65 3.19 3.69 4.2 4.76 5.53 6.25
6-hr:1.98 2.33 2.81 3.3 3.96 4.57 5.2 5.9 6.88 7.8
12-hr:2.59 3.06 3.66 4.26 5.01 5.68 6.35 7.06 8.02 8.89
24-hr:3.16 3.78 4.66 5.36 6.35 7.17 8.04 8.96 10.3 11.4
2-day:3.83 4.57 5.57 6.38 7.51 8.42 9.38 10.4 11.8 13
3-day:4.16 4.95 5.99 6.82 7.96 8.88 9.83 10.8 12.2 13.4
4-day:4.49 5.33 6.41 7.26 8.42 9.34 10.3 11.3 12.7 13.8
7-day:5.43 6.45 7.74 8.77 10.2 11.3 12.5 13.7 15.4 16.7
10-day:6.27 7.42 8.81 9.93 11.4 12.7 13.9 15.2 16.9 18.3
20-day:8.62 10.1 11.8 13 14.7 15.9 17.1 18.3 19.9 21.1
30-day:10.5 12.3 14.1 15.5 17.2 18.4 19.6 20.8 22.2 23.3
45-day:13.4 15.6 17.6 19.1 20.9 22.1 23.3 24.3 25.6 26.5
60-day:16.1 18.8 21 22.6 24.5 25.9 27.1 28.2 29.5 30.4
Date/time (GMT): Wed Dec 9 14:49:50 2020
pyRunTime: 0.026328086853

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client: NCEM Date: 12/10/2020
Project: Overtopping Study Engineer: E.Okorie
Dam: Bryson City Water Supply Dam Dam No. SWAIN-003
Reservior Water
Level (RWS)
Elevation (ft)
Primary 1 spillway
discharge (cfs)
(from HydroCAD)
Emergency spillway
discharge (cfs)
(from HydroCAD)
Total discharge from the
spillway system (cfs)
2306.6 0.0 0.0 0.0
2307.1 7.1 0.0 7.1
2307.6 15.0 0.0 15.0
2308.1 15.0 0.0 15.0
2308.6 15.1 0.0 15.1
2309.1 15.2 0.0 15.2
2309.5 15.3 0.0 15.3
2310.0 15.4 0.0 15.4
2310.5 15.4 0.0 15.4
2311.0 15.5 0.0 15.5
2311.5 15.6 0.0 15.6
2312.0 15.7 0.0 15.7
2312.5 15.8 0.0 15.8
2313.0 15.8 0.0 15.8
2313.5 15.9 0.0 15.9
2314.0 16.0 0.0 16.0
2314.4 16.1 0.0 16.1
2314.9 16.1 0.0 16.1
2315.4 16.2 0.0 16.2
2315.9 16.3 0.0 16.3
2316.4 16.4 0.0 16.4
2316.9 16.4 0.0 16.4
2317.4 16.5 0.0 16.5
2317.9 16.6 28.8 45.4
2318.4 16.7 136.1 152.8
2318.9 16.7 287.1 303.9
2319.3 16.8 471.6 488.4
2319.8 16.9 684.2 701.1
2320.3 16.9 921.7 938.6
2320.8 17.0 1073.2 1090.2
2321.3 17.1 1082.8 1099.9
2321.8 17.2 1092.4 1109.5
2322.3 17.2 1101.9 1119.1
2322.8 17.3 1111.3 1128.6
2323.3 17.4 1120.6 1138.0
2323.8 17.4 1129.9 1147.3
2324.2 17.5 1139.0 1156.5
2324.7 17.6 1148.1 1165.7
2325.2 17.6 1157.2 1174.8
2325.7 17.7 1166.1 1183.8
2326.2 17.8 1175.0 1192.8
2326.7 17.8 1183.8 1201.7
2327.2 17.9 1192.6 1210.5
2327.7 18.0 1201.3 1219.3
2328.2 18.1 1209.9 1228.0
2328.7 18.1 1218.5 1236.6
2329.1 18.2 1227.0 1245.2
2329.6 18.3 1235.4 1253.7
2330.1 18.3 1243.8 1262.1
2330.6 18.4 1252.2 1270.5
2331.1 18.4 1260.4 1278.9
2331.6 18.5 1268.7 1287.2
2332.1 18.6 1276.8 1295.4
2332.6 18.6 1285.0 1303.6
2333.1 18.7 1293.0 1311.7
2333.6 18.8 1301.1 1319.8
2334.0 18.8 1309.0 1327.9
2334.5 18.9 1317.0 1335.9
2335.0 19.0 1324.8 1343.8
2335.5 19.0 1332.7 1351.7
2336.0 19.1 1340.4 1359.5
0.0
200.0
400.0
600.0
800.0
1000.0
1200.0
1400.0
1600.0
2305.0 2310.0 2315.0 2320.0 2325.0 2330.0 2335.0 2340.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)