STOKE-102 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
LITTLE YADKIN RIVER WS DAM #18
STOKE-102
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 #18 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 =
Primary
Auxiliary Grass Lined Trapezoidal channel
192.8
152.2
158.6
165.2
171.9
178.7
185.7
1094.1
1094.6
1095.1
1095.6
1096.1
ELEVATION
1091.6
1092.1
1092.6
1093.1
1093.6
13.8
STORAGE (ACRE-FT)
0.0
1.3
2.6
4.0
5.4
6.9
8.5
1075.6
1072.6
1073.1
1073.6
1074.1
1074.6
1075.1
1071.1
1080.9
ELEVATION
1071.1
1071.6
1072.1
STORAGE (ACRE-FT)
133.9
139.8
146.0
10.2
12.0
SPILLWAY TYPE
Riser / Barrel
ELEVATION
24 HOUR - 200 YEAR EVENT, 9.95 INCHES, ELEVATION 1082.5 (1 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.77 INCHES, ELEVATION 1081.9 (1.6 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.69 INCHES, ELEVATION 1081.4 (2.1 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.68 INCHES, ELEVATION 1081 (2.5 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.33
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LITTLE YADKIN RIVER WS DAM #18, STOKE-102
ANALYSIS PERFORMED BY: S. MURPHY, HDR
24 HOUR - 1000 YEAR EVENT, 13.2 INCHES, ELEVATION 1084.1 (0.6 FT OVER-TOPPED)
TOP OF DAM ELEVATION = 1083.5
24 HOUR - 500 YEAR EVENT, 11.7 INCHES, ELEVATION 1083.4 (0.1 FT FREE-BOARD)
63.09
25
SCS Type 2
STOKE-102LITTLE YADKIN RIVER WS DAM #18 OCTOBER 5, 2020
510.7
NOT TO SCALE
429.9
442.7
455.7
469.1
482.7
496.6
358.4
369.7
381.3
393.1
405.1
417.4
295.2
305.1
315.4
325.8
336.5
347.3
239.8
248.5
257.4
266.5
275.8
285.4
200.1
207.5
215.1
223.0
231.3
1111.1
1106.1
1106.6
1107.1
1107.6
1108.1
1108.6
1109.1
1109.6
1110.1
1103.1
1103.6
1104.1
1104.6
1105.1
1105.6
1100.1
1099.1
1099.6
1096.6
1110.6
1100.6
1101.1
1101.6
1102.1
1102.6
1097.1
1097.6
1098.1
116.8
122.3
128.0
81.9
86.5
91.2
96.0
101.0
106.1
57.3
61.0
64.9
69.0
73.1
77.5
37.7
40.7
1098.6
50.3
53.7
22.1
24.4
26.9
29.4
32.1
34.8
111.4
15.8
17.8
19.9
1090.6
1091.1
1087.6
1088.1
1088.6
1089.1
1089.6
1090.1
1084.6
1085.1
43.8
47.0
1085.6
1086.1
1086.6
1087.1
1081.6
1082.1
1082.6
1083.1
1083.6
1084.1
1078.6
1079.1
1079.6
1080.1
1080.6
1081.1
1076.1
1076.6
1077.1
1077.6
1078.1
STOKE-102LITTLE YADKIN RIVER WS DAM #18 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 #18 - STOKE-102 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.25 0.50.125 Miles
Date: 10 /5/2020
/Drainage Area: 0.33 Square MilesFlowpath Length: 4737.0 feet

APPENDIX B

Lag Time Calculation
Dam No:STOKE-102 Date:10/9/2020
Dam Name:Little Yadkin River WS Dam #18 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 =63.0907 from NRCS CN Calculation
S =5.85021 in.
Mean Basin Slope Y =18.7187 %from Watershed Mean slope
Longest Flowpath:0.9 mi.from Watershed Longest flowpath
Flow Length l =4752 ft.
Time of Concentration Tc =0.68 Hours Tc =41 mins
Lag Time TL =0.41 Hours TL =25 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 8573 100 857314 1801 100 180119 67135 100 6713546
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 270511 69 18665285 86039 79 6797078 0 84 0
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 9689 72 697589 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 1469650 36 52907403 2934787 60 176087204 1485208 73 108420169 20088 79 1586986
Evergreen Forest Woods - Fair 0 36 0 47434 60 2846014 19 73 1421 991 79 78255
Mixed Forest Woods - Fair 0 36 0 304614 60 18276867 234935 73 17150270 21533 79 1701120
Shrub/Scrub Brush - Fair 0 35 0 0 56 0 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 128617 71 9131818 3367 81 272737 4830 89 429869
Hay/Pasture Pasture - Fair 0 49 0 882465 69 60890104 1216448 79 96099360 26964 84 2264958
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=63.09
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: 36.374711°
Longitude: -80.337348°
Elevation (USGS): 1128.96 ft
PRECIPITATION FREQUENCY ESTIMATES
by
duration
for ARI
(years):1 2 5 10 25 50 100 200 500 1000
5-min:0.368 0.438 0.519 0.575 0.644 0.691 0.736 0.777 0.826 0.862
10-min:0.587 0.7 0.831 0.92 1.03 1.1 1.17 1.23 1.31 1.36
15-min:0.734 0.881 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 1.91 2.2 2.56 2.84 3.12 3.39 3.75 4.03
2-hr:1.52 1.85 2.33 2.7 3.2 3.59 3.99 4.4 4.95 5.4
3-hr:1.64 2.01 2.54 2.94 3.49 3.92 4.36 4.81 5.43 5.92
6-hr:2.05 2.49 3.14 3.65 4.36 4.94 5.54 6.17 7.06 7.78
12-hr:2.53 3.08 3.89 4.54 5.46 6.22 7.04 7.92 9.18 10.2
24-hr:3.03 3.67 4.66 5.49 6.68 7.69 8.77 9.95 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 4.59 5.8 6.77 8.14 9.28 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 7.63 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 19.9 21.4 22.7 24.5 25.8
60-day:13 15.3 17.5 19.2 21.3 22.8 24.3 25.8 27.6 28.9
Date/time (GMT): Fri Oct 9 14:15:28 2020
pyRunTime: 0.0688288211823

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 #18 Dam No.STOKE-102
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
1071.1 0.0 0.0 0.0
1071.6 3.6 0.0 3.6
1072.1 3.8 0.0 3.8
1072.6 3.9 0.0 3.9
1073.1 3.9 0.0 3.9
1073.6 4.0 0.0 4.0
1074.1 4.0 0.0 4.0
1074.6 4.1 0.0 4.1
1075.1 4.1 0.0 4.1
1075.6 4.2 0.0 4.2
1076.1 4.2 0.0 4.2
1076.6 4.3 0.0 4.3
1077.1 4.3 0.0 4.3
1077.6 4.3 0.0 4.3
1078.1 4.4 0.0 4.4
1078.6 4.4 0.0 4.4
1079.1 4.5 0.0 4.5
1079.6 4.5 0.0 4.5
1080.1 4.6 0.0 4.6
1080.6 4.6 0.0 4.6
1081.1 4.7 5.6 10.3
1081.6 4.7 39.8 44.5
1082.1 4.7 95.7 100.4
1082.6 4.8 172.1 176.9
1083.1 4.8 269.3 274.1
1083.6 4.9 387.7 392.6
1084.1 4.9 528.2 533.1
1084.6 4.9 691.4 696.3
1085.1 5.0 878.1 883.1
1085.6 5.0 1089.2 1094.2
1086.1 5.1 1325.3 1330.4
1086.6 5.1 1587.4 1592.5
1087.1 5.1 1876.0 1881.2
1087.6 5.2 2192.0 2197.2
1088.1 5.2 2536.1 2541.3
1088.6 5.3 2909.0 2914.2
1089.1 5.3 3311.3 3316.6
1089.6 5.3 3743.9 3749.2
1090.1 5.4 4207.2 4212.6
1090.6 5.4 4702.1 4707.5
1091.1 5.4 5229.1 5234.6
1091.6 5.5 5788.9 5794.4
1092.1 5.5 6382.1 6387.6
1092.6 5.6 7009.3 7014.9
1093.1 5.6 7671.2 7676.7
1093.6 5.6 8368.2 8373.8
1094.1 5.7 9101.0 9106.7
1094.6 5.7 9870.2 9875.9
1095.1 5.7 10676.3 10682.0
1095.6 5.8 11519.8 11525.6
1096.1 5.8 12401.4 12407.3
1096.6 5.8 13321.6 13327.5
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
14000.0
1065.0 1070.0 1075.0 1080.0 1085.0 1090.0 1095.0 1100.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)