STOKE-103 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
LITTLE YADKIN RIVER WS DAM #20
STOKE-103
STOKES COUNTY, NC
PREPARED BY:
HDR Engineering Inc. of the
Carolinas under contract with
WOOD E&I Solutions
2020

EXECUTIVE SUMMARY
A hydraulic capacity analysis was completed for the Little Yadkin River WS Dam #20 in Stokes
County, North Carolina. The purpose of the study was to analyze the existing potential of the dam
for overtopping during predetermined specified rainfall events using the North Carolina Dam
Safety defined process and templates. 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 25 years, 50 years, 100 years, and 200
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 =
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LITTLE YADKIN RIVER WS DAM #20, STOKE-103
ANALYSIS PERFORMED BY: S. MURPHY, HDR
24 HOUR - 1000 YEAR EVENT, 13.2 INCHES, ELEVATION 1088.8 (0.8 FT OVER-TOPPED)
24 HOUR - 500 YEAR EVENT, 11.7 INCHES, ELEVATION 1088.4 (0.4 FT OVER-TOPPED)
TOP OF DAM ELEVATION = 1088
68.46
26
SCS Type 2
SPILLWAY TYPE ELEVATION
24 HOUR - 200 YEAR EVENT, 9.96 INCHES, ELEVATION 1087.7 (0.3 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.77 INCHES, ELEVATION 1087.1 (0.9 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.69 INCHES, ELEVATION 1086.6 (1.4 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.68 INCHES, ELEVATION 1086.1 (1.9 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.27
1079.0
1079.5
1080.0
1080.5
1081.0
1081.5
1077.5
1085.5
ELEVATION
1077.5
1078.0
1078.5
STORAGE (ACRE-FT)
113.9
118.8
123.8
1082.0
7.8
9.3
11.0
STORAGE (ACRE-FT)
0.0
0.6
1.5
2.5
3.6
4.9
6.3
ELEVATION
1098.0
1098.5
1099.0
1099.5
1100.0
1100.5
1101.0
1101.5
1102.0
1102.5
128.9
134.1
139.4
144.9
150.4
156.1
161.9
Primary
Auxiliary
Riser / Barrel
Grass Lined Trapezoidal channel
STOKE-103LITTLE YADKIN RIVER WS DAM #20 OCTOBER 5, 2020
1085.0
1085.5
1086.0
1086.5
1087.0
1087.5
1082.5
1083.0
1083.5
1084.0
1084.5
1092.0
1092.5
1093.0
1093.5
1088.0
1088.5
1089.0
1089.5
1090.0
1090.5
12.7
14.5
16.5
1097.0
1097.5
1094.0
1094.5
1095.0
1095.5
1096.0
1096.5
1091.0
1091.5
37.7
40.5
43.4
46.4
18.5
20.5
22.7
25.0
27.3
29.8
95.3
99.8
104.4
109.1
70.5
74.4
78.4
82.5
86.6
90.9
49.6
52.8
56.1
59.6
63.1
66.8
32.3
34.9
1105.0
1105.5
1106.0
1103.0
1117.0
1117.5
1112.5
1113.0
1113.5
1114.0
1114.5
1115.0
1115.5
1116.0
1116.5
1109.5
1110.0
1110.5
1111.0
1111.5
1112.0
1106.5
1107.0
1107.5
1108.0
1108.5
1109.0
1103.5
1104.0
1104.5
276.4
198.8
205.3
211.9
218.6
225.4
232.4
167.8
173.8
179.9
186.1
192.4
389.5
NOT TO SCALE
334.1
342.9
351.9
361.1
370.4
379.9
284.2
292.2
300.3
308.5
316.9
325.4
239.4
246.6
253.9
261.3
268.8
STOKE-103LITTLE YADKIN RIVER WS DAM #20 OCTOBER 5, 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 theGIS User Community
Little Yadkin River WS Dam #20 - STOKE-103 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.25 0.50.125 Miles
Date: 10 /5/2020
/Drainage Area: 0.27 Square MilesFlowpath Length: 1.1 miles

APPENDIX B

Lag Time Calculation
Dam No:STOKE-103 Date:10/5/2020
Dam Name:Little Yadkin River WS Dam #20 Engineer:K. Trefzger
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 =68.4566 from NRCS CN Calculation
S =4.60779 in.
Mean Basin Slope Y =16.0414 %from Watershed Mean slope
Longest Flowpath:1.05 mi.from Watershed Longest flowpath
Flow Length l =5544 ft.
Time of Concentration Tc =0.72 Hours Tc =43 mins
Lag Time TL =0.43 Hours TL =26 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 35511 100 3551084 12731 100 1273089 76602 100 7660241
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 277591 69 19153779 236738 79 18702327 0 84 0
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 0 72 0 2492 81 201855 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 75574 86 6499368 14412 91 1311530 0 94 0
Deciduous Forest Woods - Fair 0 36 0 2213943 60 132836567 2254921 73 164609209 42301 79 3341791
Evergreen Forest Woods - Fair 0 36 0 216000 60 12959998 228643 73 16690956 15990 79 1263232
Mixed Forest Woods - Fair 0 36 0 664122 60 39847337 446129 73 32567391 23442 79 1851939
Shrub/Scrub Brush - Fair 0 35 0 54244 56 3037645 9483 70 663815 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 4639 71 329362 1636 81 132512 0 89 0
Hay/Pasture Pasture - Fair 0 49 0 261901 69 18071172 361538 79 28561539 25767 84 2164421
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=68.46
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 Analysis

0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
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): King, North Carolina USA
Station Name: -
Latitude: 36.378165°
Longitude: -80.330714°
Elevation (USGS): 1134.14 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.368 0.439 0.519 0.575 0.644 0.691 0.736 0.777 0.826 0.862
10-min:0.588 0.702 0.831 0.92 1.03 1.1 1.17 1.23 1.31 1.36
15-min:0.735 0.882 1.05 1.16 1.3 1.39 1.48 1.55 1.64 1.7
30-min:1.01 1.22 1.49 1.69 1.93 2.1 2.26 2.42 2.62 2.76
60-min:1.26 1.53 1.92 2.2 2.57 2.84 3.12 3.39 3.75 4.03
2-hr:1.52 1.85 2.34 2.7 3.2 3.59 3.99 4.4 4.96 5.4
3-hr:1.65 2.01 2.54 2.94 3.49 3.92 4.36 4.81 5.43 5.93
6-hr:2.05 2.5 3.15 3.66 4.36 4.94 5.54 6.17 7.06 7.79
12-hr:2.53 3.08 3.89 4.54 5.47 6.23 7.05 7.92 9.19 10.2
24-hr:3.03 3.67 4.66 5.49 6.68 7.69 8.77 9.96 11.7 13.2
2-day:3.59 4.34 5.49 6.41 7.72 8.8 9.96 11.2 13 14.5
3-day:3.8 4.6 5.8 6.77 8.14 9.27 10.5 11.8 13.6 15.2
4-day:4.01 4.85 6.11 7.12 8.56 9.75 11 12.4 14.3 15.9
7-day:4.56 5.48 6.79 7.85 9.34 10.6 11.8 13.2 15.2 16.8
10-day:5.2 6.23 7.62 8.74 10.3 11.5 12.8 14.2 16.1 17.7
20-day:7.02 8.35 10 11.3 13.1 14.5 15.9 17.4 19.3 20.8
30-day:8.69 10.3 12.1 13.4 15.2 16.5 17.8 19.1 20.8 22.1
45-day:11 12.9 15 16.5 18.5 20 21.4 22.8 24.5 25.9
60-day:13 15.3 17.5 19.2 21.3 22.9 24.3 25.8 27.6 28.9
Date/time (GMT): Fri Oct 9 14:28:05 2020
pyRunTime: 0.0166280269623

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client:NCEM Date:10/1/2020
Project:Overtopping Study Engineer:T.Carter
Dam:Little Yadkin River WS Dam #20 Dam No.STOKE-103
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
1077.5 0.0 0.0 0.0
1078.0 3.6 0.0 3.6
1078.5 3.8 0.0 3.8
1079.0 3.8 0.0 3.8
1079.5 3.9 0.0 3.9
1080.0 4.0 0.0 4.0
1080.5 4.0 0.0 4.0
1081.0 4.1 0.0 4.1
1081.5 4.1 0.0 4.1
1082.0 4.2 0.0 4.2
1082.5 4.2 0.0 4.2
1083.0 4.3 0.0 4.3
1083.5 4.3 0.0 4.3
1084.0 4.4 0.0 4.4
1084.5 4.4 0.0 4.4
1085.0 4.5 0.0 4.5
1085.5 4.5 0.0 4.5
1086.0 4.6 31.4 35.9
1086.5 4.6 93.7 98.3
1087.0 4.7 181.3 186.0
1087.5 4.7 293.3 298.0
1088.0 4.8 429.7 434.4
1088.5 4.8 590.8 595.6
1089.0 4.8 777.2 782.1
1089.5 4.9 989.6 994.5
1090.0 4.9 1228.6 1233.5
1090.5 5.0 1494.8 1499.8
1091.0 5.0 1789.0 1794.1
1091.5 5.1 2112.0 2117.0
1092.0 5.1 2464.2 2469.3
1092.5 5.2 2846.6 2851.7
1093.0 5.2 3259.6 3264.8
1093.5 5.2 3704.1 3709.3
1094.0 5.3 4180.6 4185.9
1094.5 5.3 4689.9 4695.2
1095.0 5.4 5232.5 5237.9
1095.5 5.4 5809.1 5814.5
1096.0 5.4 6420.3 6425.7
1096.5 5.5 7066.7 7072.2
1097.0 5.5 7749.0 7754.5
1097.5 5.6 8467.6 8473.2
1098.0 5.6 9223.3 9228.9
1098.5 5.6 10016.6 10022.2
1099.0 5.7 10848.0 10853.7
1099.5 5.7 11718.1 11723.8
1100.0 5.7 12627.5 12633.3
1100.5 5.8 13576.7 13582.5
1101.0 5.8 14566.3 14572.1
1101.5 5.9 15596.8 15602.7
1102.0 5.9 16668.7 16674.6
1102.5 5.9 17782.5 17788.5
1103.0 6.0 18938.8 18944.8
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
14000.0
16000.0
18000.0
20000.0
1075.0 1080.0 1085.0 1090.0 1095.0 1100.0 1105.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)