STOKE-099 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
LITTLE YADKIN RIVER WS DAM #9
STOKE-099
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 #9 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 =
256.5
201.2
209.8
218.6
227.7
237.1
246.6
1020.1
1020.6
1021.1
1021.6
1022.1
ELEVATION
1017.6
1018.1
1018.6
1019.1
1019.6
14.4
17.0
19.7
STORAGE (ACRE-FT)
0.0
1.5
3.3
5.3
7.4
9.6
11.9
1001.6
998.6
999.1
999.6
1000.1
1000.6
1001.1
997.1
1005.6
ELEVATION
997.1
997.6
998.1
STORAGE (ACRE-FT)
176.8
184.7
192.9
72.03
28
SCS Type 2
SPILLWAY
Primary
Auxiliary
TYPE
Riser / Barrel
Grass Lined Trapezoidal channel
ELEVATION
24 HOUR - 200 YEAR EVENT, 9.84 INCHES, ELEVATION 1007.1 (1.7 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.68 INCHES, ELEVATION 1006.5 (2.3 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.61 INCHES, ELEVATION 1006.1 (2.7 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.62 INCHES, ELEVATION 1005.8 (3 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.3
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LITTLE YADKIN RIVER WS DAM #9, STOKE-099
ANALYSIS PERFORMED BY: S. MURPHY, HDR
TOP OF DAM ELEVATION = 1008.8
24 HOUR - 1000 YEAR EVENT, 13 INCHES, ELEVATION 1008.4 (0.4 FT FREE-BOARD)
24 HOUR - 500 YEAR EVENT, 11.5 INCHES, ELEVATION 1007.8 (1 FT FREE-BOARD)
STOKE-099LITTLE YADKIN RIVER WS DAM #9 OCTOBER 20, 2020
676.2
NOT TO SCALE
571.5
588.1
605.1
622.4
640.0
657.9
478.1
492.9
508.0
523.4
539.1
555.2
394.8
408.0
421.5
435.3
449.3
463.5
321.0
332.6
344.6
356.7
369.1
381.8
266.6
277.0
287.6
298.5
309.6
1035.1
1035.6
1036.1
1036.6
1037.1
1032.1
1032.6
1033.1
1033.6
1034.1
1034.6
1029.1
1029.6
1030.1
1030.6
1031.1
1031.6
1026.1
1026.6
1027.1
1027.6
1028.1
1028.6
1023.1
1023.6
1024.1
1024.6
1025.1
1025.6
1022.6
147.3
154.4
161.6
169.1
109.3
115.1
121.1
127.3
133.8
140.4
77.8
82.6
87.6
92.8
98.2
103.6
52.4
56.3
60.2
64.4
68.7
73.1
31.5
34.7
38.1
41.5
45.0
48.7
22.5
25.4
28.4
1016.6
1017.1
1013.6
1014.1
1014.6
1015.1
1015.6
1016.1
1010.6
1011.1
1011.6
1012.1
1012.6
1013.1
1007.6
1008.1
1008.6
1009.1
1009.6
1010.1
1004.6
1005.1
1005.6
1006.1
1006.6
1007.1
1002.1
1002.6
1003.1
1003.6
1004.1
STOKE-099LITTLE YADKIN RIVER WS DAM #9 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 #9 - STOKE-099 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.2 0.40.1 Miles
Date: 10 /5/2020
/Drainage Ar ea: 0.3 Square MilesFlowpath Length: 1.0 miles

APPENDIX B

Lag Time Calculation
Dam No:STOKE-099 Date:10/20/2020
Dam Name:Little Yadkin River WS Dam #9 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 =72.0253 from NRCS CN Calculation
S =3.88401 in.
Mean Basin Slope Y =11.1838 %from Watershed Mean slope
Longest Flowpath:1.02 mi.from Watershed Longest flowpath
Flow Length l =5385.6 ft.
Time of Concentration Tc =0.77 Hours Tc =46 mins
Lag Time TL =0.46 Hours TL =28 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 6398 100 639757 7288 100 728828 63823 100 6382298
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 382683 69 26405124 507817 79 40117526 0 84 0
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 0 72 0 31097 81 2518887 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 0 36 0 769700 60 46182030 1056668 73 77136744 44082 79 3482516
Evergreen Forest Woods - Fair 0 36 0 236039 60 14162353 147986 73 10802986 0 79 0
Mixed Forest Woods - Fair 0 36 0 636452 60 38187111 858281 73 62654501 941 79 74336
Shrub/Scrub Brush - Fair 0 35 0 20609 56 1154076 56471 70 3952971 429 77 33043
Herbaceuous Herbaceuous - Fair 0 60 0 14328 71 1017274 128673 81 10422551 19158 89 1705085
Hay/Pasture Pasture - Fair 0 49 0 1380361 69 95244905 1811153 79 143081063 9997 84 839734
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 49186 80 3934886 173652 87 15107691 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=72.03
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: 36.351375°
Longitude: -80.360726°
Elevation (USGS): 1059.7 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 0.824 0.86
10-min:0.587 0.7 0.829 0.919 1.02 1.1 1.17 1.23 1.3 1.35
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 1.69 1.92 2.1 2.26 2.42 2.61 2.75
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 1.84 2.33 2.69 3.19 3.57 3.97 4.37 4.93 5.36
3-hr:1.64 2 2.52 2.92 3.47 3.9 4.34 4.78 5.4 5.89
6-hr:2.04 2.48 3.12 3.63 4.33 4.9 5.5 6.12 7 7.72
12-hr:2.5 3.05 3.85 4.5 5.41 6.17 6.98 7.84 9.1 10.1
24-hr:3 3.63 4.62 5.44 6.62 7.61 8.68 9.84 11.5 13
2-day:3.56 4.3 5.43 6.35 7.64 8.7 9.84 11.1 12.8 14.3
3-day:3.77 4.55 5.74 6.7 8.05 9.17 10.4 11.6 13.5 15
4-day:3.98 4.8 6.05 7.05 8.47 9.64 10.9 12.2 14.1 15.7
7-day:4.52 5.43 6.73 7.77 9.24 10.4 11.7 13 14.9 16.5
10-day:5.16 6.18 7.56 8.66 10.2 11.4 12.7 14 15.9 17.4
20-day:6.96 8.28 9.93 11.2 13 14.4 15.7 17.2 19.1 20.6
30-day:8.62 10.2 12 13.3 15 16.4 17.6 18.9 20.6 21.8
45-day:10.9 12.8 14.8 16.3 18.3 19.7 21.1 22.5 24.2 25.5
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 14:15:13 2020
pyRunTime: 0.0456240177155

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 #9 Dam No.STOKE-099
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
997.1 0.0 0.0 0.0
997.6 3.6 0.0 3.6
998.1 4.0 0.0 4.0
998.6 4.1 0.0 4.1
999.1 4.1 0.0 4.1
999.6 4.2 0.0 4.2
1000.1 4.2 0.0 4.2
1000.6 4.3 0.0 4.3
1001.1 4.3 0.0 4.3
1001.6 4.4 0.0 4.4
1002.1 4.4 0.0 4.4
1002.6 4.4 0.0 4.4
1003.1 4.5 0.0 4.5
1003.6 4.5 0.0 4.5
1004.1 4.6 0.0 4.6
1004.6 4.6 0.0 4.6
1005.1 4.7 0.0 4.7
1005.6 4.7 0.0 4.7
1006.1 4.7 33.4 38.2
1006.6 4.8 99.2 103.9
1007.1 4.8 190.6 195.5
1007.6 4.9 306.6 311.5
1008.1 4.9 446.8 451.7
1008.6 4.9 611.3 616.3
1009.1 5.0 800.6 805.6
1009.6 5.0 1015.2 1020.2
1010.1 5.0 1255.5 1260.6
1010.6 5.1 1522.2 1527.3
1011.1 5.1 1815.8 1820.9
1011.6 5.2 2136.9 2142.1
1012.1 5.2 2486.2 2491.4
1012.6 5.2 2864.2 2869.4
1013.1 5.3 3271.4 3276.7
1013.6 5.3 3708.6 3713.9
1014.1 5.3 4176.3 4181.7
1014.6 5.4 4675.1 4680.4
1015.1 5.4 5205.4 5210.8
1015.6 5.4 5768.0 5773.4
1016.1 5.5 6363.3 6368.8
1016.6 5.5 6991.9 6997.5
1017.1 5.6 7654.4 7660.0
1017.6 5.6 8351.3 8356.8
1018.1 5.6 9083.0 9088.6
1018.6 5.7 9850.2 9855.8
1019.1 5.7 10653.3 10659.0
1019.6 5.7 11492.9 11498.6
1020.1 5.8 12369.4 12375.1
1020.6 5.8 13283.4 13289.2
1021.1 5.8 14235.3 14241.1
1021.6 5.9 15225.6 15231.5
1022.1 5.9 16254.8 16260.7
1022.6 5.9 17323.4 17329.4
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
14000.0
16000.0
18000.0
20000.0
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)