STOKE-100 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
LITTLE YADKIN RIVER WS DAM #13A
STOKE-100
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 #13a 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, 200 years,
and 500 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 =
301.5
243.4
252.5
261.9
271.5
281.3
291.3
1048.8
1049.3
1049.8
1050.3
1050.8
ELEVATION
1046.3
1046.8
1047.3
1047.8
1048.3
21.1
24.5
28.0
STORAGE (ACRE-FT)
0.0
2.8
5.7
8.6
11.6
14.7
17.8
1030.3
1027.3
1027.8
1028.3
1028.8
1029.3
1029.8
1025.8
1036.1
ELEVATION
1025.8
1026.3
1026.8
STORAGE (ACRE-FT)
217.0
225.6
234.4
60.24
48
SCS Type 2
SPILLWAY
Primary
Auxiliary
TYPE
Riser / Barrel
Grass Lined Trapezoidal channel
ELEVATION
24 HOUR - 200 YEAR EVENT, 9.92 INCHES, ELEVATION 1037.7 (1.2 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.74 INCHES, ELEVATION 1037.1 (1.8 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.66 INCHES, ELEVATION 1036.6 (2.3 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.66 INCHES, ELEVATION 1036.3 (2.6 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.71
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LITTLE YADKIN RIVER WS DAM #13A, STOKE-100
ANALYSIS PERFORMED BY: S. MURPHY, HDR
24 HOUR - 1000 YEAR EVENT, 13.1 INCHES, ELEVATION 1039.3 (0.4 FT OVER-TOPPED)
TOP OF DAM ELEVATION = 1038.9
24 HOUR - 500 YEAR EVENT, 11.6 INCHES, ELEVATION 1038.6 (0.3 FT FREE-BOARD)
STOKE-100LITTLE YADKIN RIVER WS DAM #13A OCTOBER 20, 2020
732.5
NOT TO SCALE
625.0
642.1
659.5
677.2
695.3
713.7
529.1
544.3
559.8
575.6
591.7
608.2
443.7
457.2
471.0
485.1
499.5
514.2
367.7
379.7
392.0
404.5
417.3
430.4
311.9
322.5
333.4
344.6
356.0
1063.8
1064.3
1064.8
1065.3
1065.8
1060.8
1061.3
1061.8
1062.3
1062.8
1063.3
1057.8
1058.3
1058.8
1059.3
1059.8
1060.3
1054.8
1055.3
1055.8
1056.3
1056.8
1057.3
1051.8
1052.3
1052.8
1053.3
1053.8
1054.3
1051.3
184.5
192.3
200.4
208.6
141.3
148.1
155.1
162.2
169.4
176.9
103.3
109.3
115.4
121.6
128.0
134.6
70.6
75.7
80.9
86.3
91.9
97.5
43.3
47.4
51.7
56.2
60.8
65.6
31.7
35.4
39.3
1045.3
1045.8
1042.3
1042.8
1043.3
1043.8
1044.3
1044.8
1039.3
1039.8
1040.3
1040.8
1041.3
1041.8
1036.3
1036.8
1037.3
1037.8
1038.3
1038.8
1033.3
1033.8
1034.3
1034.8
1035.3
1035.8
1030.8
1031.3
1031.8
1032.3
1032.8
STOKE-100LITTLE YADKIN RIVER WS DAM #13A OCTOBER 20, 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 #13a - STOKE-100 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.35 0.70.175 Miles
Date: 10 /5/2020
/Drainage Area: 0.71 Square MilesFlowpath Length: 2.0 miles

APPENDIX B

Lag Time Calculation
Dam No:STOKE-100 Date:10/20/2020
Dam Name:Little Yadkin River WS Dam #13a 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 =60.237 from NRCS CN Calculation
S =6.6011 in.
Mean Basin Slope Y =20.4022 %from Watershed Mean slope
Longest Flowpath:2.02 mi.from Watershed Longest flowpath
Flow Length l =10665.6 ft.
Time of Concentration Tc =1.34 Hours Tc =80 mins
Lag Time TL =0.80 Hours TL =48 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 17003 100 1700304 3301 100 330077 154091 100 15409101
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 552679 69 38134831 275148 79 21736728 0 84 0
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 47835 72 3444100 25024 81 2026964 0 86 0
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 9687 85 823423 1 90 122 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 4431735 36 159542470 6360529 60 381631714 1229297 73 89738682 37654 79 2974682
Evergreen Forest Woods - Fair 0 36 0 829589 60 49775352 608967 73 44454557 2986 79 235902
Mixed Forest Woods - Fair 7450 36 268203 952533 60 57152006 471872 73 34446641 15250 79 1204731
Shrub/Scrub Brush - Fair 4 35 141 95577 56 5352331 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 22568 71 1602293 92 81 7452 6406 89 570172
Hay/Pasture Pasture - Fair 0 49 0 1265540 69 87322242 1163201 79 91892875 76883 84 6458178
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 766052 80 61284141 354997 87 30884736 44346 90 3991097
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=60.24
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: 36.368762°
Longitude: -80.354462°
Elevation (USGS): 1153.25 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.367 0.438 0.518 0.575 0.643 0.69 0.735 0.776 0.825 0.861
10-min:0.587 0.7 0.829 0.919 1.03 1.1 1.17 1.23 1.31 1.36
15-min:0.734 0.88 1.05 1.16 1.3 1.39 1.48 1.55 1.64 1.7
30-min:1.01 1.22 1.49 1.68 1.92 2.1 2.26 2.42 2.61 2.76
60-min:1.25 1.53 1.91 2.19 2.56 2.84 3.11 3.39 3.75 4.02
2-hr:1.51 1.85 2.33 2.7 3.19 3.58 3.98 4.39 4.94 5.38
3-hr:1.64 2.01 2.53 2.93 3.48 3.91 4.35 4.8 5.42 5.91
6-hr:2.04 2.49 3.13 3.64 4.35 4.92 5.52 6.15 7.03 7.76
12-hr:2.52 3.06 3.87 4.52 5.44 6.2 7.02 7.89 9.15 10.2
24-hr:3.02 3.66 4.65 5.47 6.66 7.66 8.74 9.92 11.6 13.1
2-day:3.58 4.33 5.47 6.39 7.7 8.77 9.92 11.2 12.9 14.4
3-day:3.79 4.58 5.78 6.75 8.11 9.24 10.4 11.7 13.6 15.1
4-day:4 4.83 6.09 7.1 8.53 9.71 11 12.3 14.2 15.8
7-day:4.55 5.47 6.77 7.82 9.3 10.5 11.8 13.2 15.1 16.7
10-day:5.19 6.22 7.6 8.72 10.3 11.5 12.8 14.2 16 17.6
20-day:7 8.33 9.99 11.3 13.1 14.4 15.9 17.3 19.2 20.7
30-day:8.67 10.2 12 13.4 15.1 16.5 17.8 19 20.7 22
45-day:10.9 12.9 14.9 16.4 18.4 19.9 21.3 22.6 24.4 25.7
60-day:13 15.3 17.5 19.1 21.2 22.8 24.2 25.7 27.5 28.8
Date/time (GMT): Tue Oct 20 15:46:57 2020
pyRunTime: 0.0623071193695

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client:NCEM Date:10/19/2020
Project:Overtopping Study Engineer:T.Carter
Dam:Little Yadkin River WS Dam #13a Dam No.STOKE-100
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
1025.8 0.0 0.0 0.0
1026.3 3.5 0.0 3.5
1026.8 3.5 0.0 3.5
1027.3 3.6 0.0 3.6
1027.8 3.6 0.0 3.6
1028.3 3.7 0.0 3.7
1028.8 3.7 0.0 3.7
1029.3 3.8 0.0 3.8
1029.8 3.8 0.0 3.8
1030.3 3.9 0.0 3.9
1030.8 3.9 0.0 3.9
1031.3 4.0 0.0 4.0
1031.8 4.0 0.0 4.0
1032.3 4.1 0.0 4.1
1032.8 4.1 0.0 4.1
1033.3 4.2 0.0 4.2
1033.8 4.2 0.0 4.2
1034.3 4.3 0.0 4.3
1034.8 4.3 0.0 4.3
1035.3 4.3 0.0 4.3
1035.8 4.4 0.0 4.4
1036.3 4.4 12.1 16.5
1036.8 4.5 81.8 86.2
1037.3 4.5 189.4 193.9
1037.8 4.6 329.4 333.9
1038.3 4.6 499.6 504.2
1038.8 4.6 699.2 703.8
1039.3 4.7 927.9 932.6
1039.8 4.7 1185.7 1190.4
1040.3 4.8 1472.7 1477.5
1040.8 4.8 1789.3 1794.1
1041.3 4.8 2135.6 2140.4
1041.8 4.9 2512.1 2517.0
1042.3 4.9 2919.3 2924.2
1042.8 5.0 3357.5 3362.5
1043.3 5.0 3827.2 3832.2
1043.8 5.0 4328.9 4333.9
1044.3 5.1 4863.0 4868.0
1044.8 5.1 5429.9 5435.1
1045.3 5.2 6030.3 6035.4
1045.8 5.2 6664.4 6669.6
1046.3 5.2 7332.9 7338.1
1046.8 5.3 8036.1 8041.4
1047.3 5.3 8774.6 8779.9
1047.8 5.3 9548.8 9554.1
1048.3 5.4 10359.2 10364.5
1048.8 5.4 11206.1 11211.6
1049.3 5.4 12090.2 12095.7
1049.8 5.5 13011.8 13017.3
1050.3 5.5 13971.4 13976.9
1050.8 5.5 14969.4 14974.9
1051.3 5.6 16006.3 16011.8
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
14000.0
16000.0
18000.0
1020.0 1025.0 1030.0 1035.0 1040.0 1045.0 1050.0 1055.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)