STOKE-104 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
LITTLE YADKIN RIVER WS DAM #25
STOKE-104
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 #25 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 =
1721.4
1443.3
1488.2
1533.7
1579.7
1626.4
1673.6
1017.0
1017.5
1018.0
1018.5
1019.0
ELEVATION
1014.5
1015.0
1015.5
1016.0
1016.5
165.1
190.4
216.2
STORAGE (ACRE-FT)
0.0
22.1
44.7
67.8
91.4
115.5
140.1
998.5
995.5
996.0
996.5
997.0
997.5
998.0
994.0
1003.5
ELEVATION
994.0
994.5
995.0
STORAGE (ACRE-FT)
1312.3
1355.4
1399.0
73.02
29
SCS Type 2
SPILLWAY
Primary
Auxiliary
TYPE
Riser / Barrel
Grass Lined Trapezoidal channel
ELEVATION
24 HOUR - 200 YEAR EVENT, 9.81 INCHES, ELEVATION 995.9 (9.9 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.65 INCHES, ELEVATION 995.6 (10.2 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.59 INCHES, ELEVATION 995.3 (10.5 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.6 INCHES, ELEVATION 995 (10.8 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.26
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LITTLE YADKIN RIVER WS DAM #25, STOKE-104
ANALYSIS PERFORMED BY: S. MURPHY, HDR
TOP OF DAM ELEVATION = 1005.8
24 HOUR - 1000 YEAR EVENT, 12.9 INCHES, ELEVATION 996.7 (9.1 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 11.5 INCHES, ELEVATION 996.3 (9.5 FT FREE-BOARD)
STOKE-104LITTLE YADKIN RIVER WS DAM #25 OCTOBER 20, 2020
3486.2
NOT TO SCALE
3079.5
3145.2
3211.8
3279.1
3347.3
3416.3
2701.2
2762.3
2824.2
2886.9
2950.3
3014.5
2349.7
2406.6
2464.1
2522.3
2581.2
2640.9
2023.1
2075.8
2129.2
2183.3
2238.1
2293.6
1769.9
1819.1
1869.0
1919.7
1971.1
1032.0
1032.5
1033.0
1033.5
1034.0
1029.0
1029.5
1030.0
1030.5
1031.0
1031.5
1026.0
1026.5
1027.0
1027.5
1028.0
1028.5
1023.0
1023.5
1024.0
1024.5
1025.0
1025.5
1020.0
1020.5
1021.0
1021.5
1022.0
1022.5
1019.5
1146.0
1186.7
1228.0
1269.8
913.0
950.5
988.6
1027.1
1066.2
1105.8
698.7
733.1
768.1
803.5
839.5
876.0
502.7
534.1
566.0
598.4
631.3
664.7
324.8
353.3
382.2
411.6
441.5
471.9
242.5
269.4
296.8
1013.5
1014.0
1010.5
1011.0
1011.5
1012.0
1012.5
1013.0
1007.5
1008.0
1008.5
1009.0
1009.5
1010.0
1004.5
1005.0
1005.5
1006.0
1006.5
1007.0
1001.5
1002.0
1002.5
1003.0
1003.5
1004.0
999.0
999.5
1000.0
1000.5
1001.0
STOKE-104LITTLE YADKIN RIVER WS DAM #25 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 #25 - STOKE-104 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.2 0.40.1 Miles
Date: 10 /5/2020
/Drainage Area: 0.26 Square MilesFlowpath Length: 1.1 miles

APPENDIX B

Lag Time Calculation
Dam No:STOKE-104 Date:10/20/2020
Dam Name:Little Yadkin River WS Dam #25 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 =73.0247 from NRCS CN Calculation
S =3.694 in.
Mean Basin Slope Y =10.0996 %from Watershed Mean slope
Longest Flowpath:1.07 mi.from Watershed Longest flowpath
Flow Length l =5649.6 ft.
Time of Concentration Tc =0.82 Hours Tc =49 mins
Lag Time TL =0.49 Hours TL =29 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 24272 100 2427233 6882 100 688169 36666 100 3666577
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 424994 69 29324585 184019 79 14537464 22709 84 1907526
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 100900 72 7264808 2558 81 207205 0 86 0
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 1970 85 167481 7718 90 694638 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 1308405 60 78504298 550821 73 40209931 56508 79 4464124
Evergreen Forest Woods - Fair 0 36 0 116083 60 6964994 203196 73 14833340 14157 79 1118376
Mixed Forest Woods - Fair 0 36 0 256705 60 15402311 332403 73 24265426 11994 79 947502
Shrub/Scrub Brush - Fair 0 35 0 0 56 0 0 70 0 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 22956 71 1629857 47478 81 3845704 15154 89 1348672
Hay/Pasture Pasture - Fair 0 49 0 1002236 69 69154308 1182814 79 93442267 425095 84 35707975
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 361726 80 28938057 256861 87 22346929 324220 90 29179802
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=73.02
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: 36.346022°
Longitude: -80.335976°
Elevation (USGS): 1041.46 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 0.736 0.776 0.825 0.86
10-min:0.589 0.703 0.832 0.921 1.03 1.1 1.17 1.23 1.31 1.36
15-min:0.736 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 2.7 3.19 3.58 3.98 4.38 4.93 5.37
3-hr:1.64 2.01 2.53 2.93 3.48 3.9 4.34 4.79 5.4 5.89
6-hr:2.04 2.48 3.13 3.63 4.34 4.91 5.5 6.13 7.01 7.72
12-hr:2.51 3.05 3.85 4.5 5.42 6.17 6.98 7.85 9.1 10.1
24-hr:3 3.63 4.62 5.43 6.6 7.59 8.65 9.81 11.5 12.9
2-day:3.56 4.3 5.43 6.33 7.62 8.68 9.81 11 12.8 14.2
3-day:3.76 4.55 5.73 6.68 8.03 9.14 10.3 11.6 13.4 14.9
4-day:3.97 4.8 6.04 7.03 8.44 9.61 10.8 12.2 14.1 15.6
7-day:4.52 5.42 6.71 7.75 9.21 10.4 11.7 13 14.9 16.5
10-day:5.15 6.17 7.54 8.64 10.2 11.4 12.7 14 15.9 17.4
20-day:6.96 8.27 9.92 11.2 13 14.3 15.7 17.2 19.1 20.6
30-day:8.61 10.2 11.9 13.3 15 16.3 17.6 18.9 20.6 21.9
45-day:10.9 12.8 14.8 16.3 18.3 19.7 21.1 22.5 24.3 25.6
60-day:12.9 15.2 17.4 19 21.1 22.6 24.1 25.5 27.3 28.6
Date/time (GMT): Tue Oct 20 17:23:27 2020
pyRunTime: 0.0342619419098

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 #25 Dam No.STOKE-104
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
994.0 0.0 0.0 0.0
994.5 3.6 0.0 3.6
995.0 3.7 0.0 3.7
995.5 3.8 0.0 3.8
996.0 3.9 0.0 3.9
996.5 3.9 0.0 3.9
997.0 4.0 0.0 4.0
997.5 4.0 0.0 4.0
998.0 4.1 0.0 4.1
998.5 4.1 0.0 4.1
999.0 4.2 0.0 4.2
999.5 4.2 0.0 4.2
1000.0 4.3 0.0 4.3
1000.5 4.3 0.0 4.3
1001.0 4.4 0.0 4.4
1001.5 4.4 0.0 4.4
1002.0 4.5 0.0 4.5
1002.5 4.5 0.0 4.5
1003.0 4.6 0.0 4.6
1003.5 4.6 0.0 4.6
1004.0 4.7 32.9 37.6
1004.5 4.7 99.3 104.0
1005.0 4.8 193.9 198.7
1005.5 4.8 316.2 321.0
1006.0 4.9 466.6 471.5
1006.5 4.9 645.9 650.8
1007.0 5.0 854.8 859.8
1007.5 5.0 1094.4 1099.4
1008.0 5.0 1365.5 1370.6
1008.5 5.1 1669.2 1674.3
1009.0 5.1 2006.4 2011.5
1009.5 5.2 2378.0 2383.1
1010.0 5.2 2784.9 2790.1
1010.5 5.3 3228.1 3233.3
1011.0 5.3 3708.4 3713.7
1011.5 5.3 4226.8 4232.1
1012.0 5.4 4784.1 4789.5
1012.5 5.4 5381.1 5386.5
1013.0 5.5 6018.7 6024.2
1013.5 5.5 6697.7 6703.2
1014.0 5.6 7418.9 7424.5
1014.5 5.6 8183.1 8188.7
1015.0 5.6 8991.1 8996.7
1015.5 5.7 9843.6 9849.3
1016.0 5.7 10741.4 10747.1
1016.5 5.7 11685.2 11691.0
1017.0 5.8 12675.8 12681.6
1017.5 5.8 13713.9 13719.7
1018.0 5.9 14800.2 14806.1
1018.5 5.9 15935.4 15941.3
1019.0 5.9 17120.2 17126.1
1019.5 6.0 18355.2 18361.2
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
14000.0
16000.0
18000.0
20000.0
990.0 995.0 1000.0 1005.0 1010.0 1015.0 1020.0 1025.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)