STOKE-105 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
LITTLE YADKIN RIVER WS DAM #26
STOKE-105
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 #26 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,
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 =
206.2
165.0
171.5
178.2
185.0
191.9
198.9
1030.3
1030.8
1031.3
1031.8
1032.3
ELEVATION
1027.8
1028.3
1028.8
1029.3
1029.8
12.4
14.6
16.9
STORAGE (ACRE-FT)
0.0
1.5
3.2
4.8
6.6
8.4
10.4
1011.8
1008.8
1009.3
1009.8
1010.3
1010.8
1011.3
1007.3
1017.8
ELEVATION
1007.3
1007.8
1008.3
STORAGE (ACRE-FT)
146.4
152.4
158.7
77.96
21
SCS Type 2
SPILLWAY
Primary
Auxiliary
TYPE
Riser / Barrel
Grass Lined Trapezoidal channel
ELEVATION
24 HOUR - 200 YEAR EVENT, 9.76 INCHES, ELEVATION 1018.4 (2.3 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.61 INCHES, ELEVATION 1018 (2.7 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.56 INCHES, ELEVATION 1017.6 (3.1 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.58 INCHES, ELEVATION 1016.1 (4.6 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.21
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LITTLE YADKIN RIVER WS DAM #26, STOKE-105
ANALYSIS PERFORMED BY: S. MURPHY, HDR
TOP OF DAM ELEVATION = 1020.7
24 HOUR - 1000 YEAR EVENT, 12.8 INCHES, ELEVATION 1019.5 (1.2 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 11.4 INCHES, ELEVATION 1019 (1.7 FT FREE-BOARD)
STOKE-105LITTLE YADKIN RIVER WS DAM #26 OCTOBER 20, 2020
499.6
NOT TO SCALE
426.7
438.4
450.2
462.3
474.5
486.9
361.2
371.3
382.0
392.8
403.9
415.2
303.9
313.1
322.4
331.8
341.5
351.2
252.4
260.6
268.9
277.4
286.1
294.9
213.5
221.0
228.6
236.4
244.3
1045.3
1045.8
1046.3
1046.8
1047.3
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
1032.8
123.4
129.0
134.7
140.5
92.6
97.4
102.4
107.5
112.7
118.0
66.3
70.3
74.5
78.8
83.3
87.9
44.7
48.0
51.5
55.0
58.6
62.4
26.8
29.6
32.4
35.3
38.4
41.5
19.2
21.6
24.2
1026.8
1027.3
1023.8
1024.3
1024.8
1025.3
1025.8
1026.3
1020.8
1021.3
1021.8
1022.3
1022.8
1023.3
1017.8
1018.3
1018.8
1019.3
1019.8
1020.3
1014.8
1015.3
1015.8
1016.3
1016.8
1017.3
1012.3
1012.8
1013.3
1013.8
1014.3
STOKE-105LITTLE YADKIN RIVER WS DAM #26 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 #26 - STOKE-105 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.2 0.40.1 Miles
Date: 10 /5/2020
/Drainage Area: 0.21 Square MilesFlowpath Length: 4101.5 feet

APPENDIX B

Lag Time Calculation
Dam No:STOKE-105 Date:10/20/2020
Dam Name:Little Yadkin River WS Dam #26 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 =77.9615 from NRCS CN Calculation
S =2.82685 in.
Mean Basin Slope Y =8.67757 %from Watershed Mean slope
Longest Flowpath:0.78 mi.from Watershed Longest flowpath
Flow Length l =4118.4 ft.
Time of Concentration Tc =0.59 Hours Tc =36 mins
Lag Time TL =0.36 Hours TL =21 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 27741 100 2774130 64254 100 6425360 92087 100 9208660
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 181281 69 12508420 492367 79 38897025 640647 84 53814330
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 38283 72 2756348 137075 81 11103063 257560 86 22150166
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 0 36 0 715154 60 42909223 663816 73 48458539 335394 79 26496100
Evergreen Forest Woods - Fair 0 36 0 80337 60 4820210 47862 73 3493922 42644 79 3368854
Mixed Forest Woods - Fair 0 36 0 118954 60 7137218 179379 73 13094638 36222 79 2861541
Shrub/Scrub Brush - Fair 0 35 0 0 56 0 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 8550 71 607080 2729 81 221081 2437 89 216930
Hay/Pasture Pasture - Fair 0 49 0 0 69 0 265666 79 20987620 716608 84 60195066
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 291775 80 23341989 32334 87 2813047 478641 90 43077696
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 9688 90 871965 0 95 0 Weighted CN=77.96
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: 36.338095°
Longitude: -80.334112°
Elevation (USGS): 1051.14 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.369 0.439 0.52 0.576 0.644 0.691 0.735 0.776 0.824 0.86
10-min:0.589 0.703 0.832 0.921 1.03 1.1 1.17 1.23 1.3 1.35
15-min:0.737 0.883 1.05 1.17 1.3 1.39 1.48 1.55 1.64 1.7
30-min:1.01 1.22 1.5 1.69 1.93 2.1 2.26 2.42 2.61 2.75
60-min:1.26 1.53 1.92 2.2 2.57 2.84 3.12 3.39 3.75 4.02
2-hr:1.51 1.85 2.33 2.69 3.19 3.58 3.97 4.37 4.92 5.36
3-hr:1.64 2 2.53 2.93 3.47 3.9 4.33 4.78 5.39 5.88
6-hr:2.04 2.48 3.12 3.63 4.33 4.9 5.49 6.11 6.99 7.7
12-hr:2.5 3.04 3.84 4.49 5.4 6.15 6.97 7.83 9.07 10.1
24-hr:2.99 3.62 4.6 5.41 6.58 7.56 8.61 9.76 11.4 12.8
2-day:3.54 4.28 5.41 6.31 7.59 8.64 9.76 11 12.7 14.1
3-day:3.75 4.53 5.71 6.66 8 9.1 10.3 11.5 13.3 14.8
4-day:3.96 4.78 6.01 7 8.41 9.56 10.8 12.1 14 15.5
7-day:4.5 5.4 6.69 7.72 9.17 10.4 11.6 12.9 14.8 16.4
10-day:5.14 6.15 7.51 8.61 10.1 11.3 12.6 13.9 15.8 17.3
20-day:6.93 8.24 9.88 11.2 12.9 14.3 15.7 17.1 19 20.5
30-day:8.58 10.1 11.9 13.2 15 16.3 17.6 18.8 20.5 21.8
45-day:10.8 12.8 14.8 16.3 18.2 19.7 21.1 22.4 24.2 25.5
60-day:12.9 15.1 17.3 18.9 21 22.5 24 25.4 27.2 28.5
Date/time (GMT): Tue Oct 20 18:32:51 2020
pyRunTime: 0.0323038101196

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client:NCEM Date:10/20/2020
Project:Overtopping Study Engineer:T.Carter
Dam:Little Yadkin River WS Dam #26 Dam No.STOKE-105
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
1007.3 0.0 0.0 0.0
1007.8 3.6 0.0 3.6
1008.3 3.7 0.0 3.7
1008.8 3.7 0.0 3.7
1009.3 3.8 0.0 3.8
1009.8 3.8 0.0 3.8
1010.3 3.9 0.0 3.9
1010.8 3.9 0.0 3.9
1011.3 4.0 0.0 4.0
1011.8 4.0 0.0 4.0
1012.3 4.1 0.0 4.1
1012.8 4.2 0.0 4.2
1013.3 4.2 0.0 4.2
1013.8 4.2 0.0 4.2
1014.3 4.3 0.0 4.3
1014.8 4.3 0.0 4.3
1015.3 4.4 0.0 4.4
1015.8 4.4 0.0 4.4
1016.3 4.5 0.0 4.5
1016.8 4.5 0.0 4.5
1017.3 4.6 0.0 4.6
1017.8 4.6 0.0 4.6
1018.3 4.7 32.1 36.8
1018.8 4.7 95.1 99.8
1019.3 4.8 182.2 187.0
1019.8 4.8 292.2 297.0
1020.3 4.8 424.6 429.5
1020.8 4.9 579.6 584.5
1021.3 4.9 757.4 762.4
1021.8 5.0 958.4 963.4
1022.3 5.0 1183.0 1188.0
1022.8 5.1 1431.6 1436.7
1023.3 5.1 1704.9 1710.0
1023.8 5.1 2003.2 2008.3
1024.3 5.2 2327.1 2332.3
1024.8 5.2 2677.1 2682.3
1025.3 5.3 3053.7 3059.0
1025.8 5.3 3457.5 3462.8
1026.3 5.3 3888.8 3894.2
1026.8 5.4 4348.4 4353.7
1027.3 5.4 4836.5 4841.9
1027.8 5.4 5353.7 5359.2
1028.3 5.5 5900.6 5906.1
1028.8 5.5 6477.5 6483.1
1029.3 5.6 7085.0 7090.6
1029.8 5.6 7723.6 7729.2
1030.3 5.6 8393.6 8399.2
1030.8 5.7 9095.5 9101.2
1031.3 5.7 9829.9 9835.6
1031.8 5.7 10597.1 10602.9
1032.3 5.8 11397.6 11403.4
1032.8 5.8 12231.8 12237.7
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
14000.0
1005.0 1010.0 1015.0 1020.0 1025.0 1030.0 1035.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)