STOKE-107 Final Report

DAM HYDRAULIC CAPACITY
ASSESSMENT REPORT
(DRAFT)
LITTLE YADKIN RIVER WS DAM # 29
STOKE-107
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 # 29 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, and 200
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 =
261.4
201.7
211.0
220.6
230.4
240.5
250.8
1007.7
1008.2
1008.7
1009.2
1009.7
ELEVATION
1005.2
1005.7
1006.2
1006.7
1007.2
14.1
16.6
19.2
STORAGE (ACRE-FT)
0.0
1.6
3.4
5.2
7.2
9.4
11.7
989.2
986.2
986.7
987.2
987.7
988.2
988.7
984.7
995.0
ELEVATION
984.7
985.2
985.7
STORAGE (ACRE-FT)
175.3
183.8
192.6
72.18
34
SCS Type 2
SPILLWAY
Primary
Auxiliary
TYPE
Riser / Barrel
Grass Lined Trapezoidal Channel
ELEVATION
24 HOUR - 200 YEAR EVENT, 9.71 INCHES, ELEVATION 996.9 (0.7 FT FREE-BOARD)
24 HOUR - 100 YEAR EVENT, 8.57 INCHES, ELEVATION 996.3 (1.3 FT FREE-BOARD)
24 HOUR - 50 YEAR EVENT, 7.52 INCHES, ELEVATION 995.8 (1.8 FT FREE-BOARD)
24 HOUR - 25 YEAR EVENT, 6.55 INCHES, ELEVATION 995.4 (2.2 FT FREE-BOARD)
SUMMARY OF INPUT PARAMETERS
0.43
SUMMARY SHEET OF HYDRAULIC CAPACITY RESULTS
LITTLE YADKIN RIVER WS DAM #29, STOKE-107
ANALYSIS PERFORMED BY: S. MURPHY, HDR
24 HOUR - 1000 YEAR EVENT, 12.8 INCHES, ELEVATION 998.2 (0.6 FT OVER-TOPPED)
24 HOUR - 500 YEAR EVENT, 11.4 INCHES, ELEVATION 997.7 (0.1 FT OVER-TOPPED)
TOP OF DAM ELEVATION = 997.6
STOKE-107LITTLE YADKIN RIVER WS DAM #29 OCTOBER 27, 2020
731.8
NOT TO SCALE
609.5
628.7
648.4
668.6
689.2
710.3
503.2
519.9
537.0
554.5
572.4
590.7
410.8
425.4
440.2
455.4
471.0
486.9
330.7
343.3
356.2
369.3
382.8
396.7
272.3
283.4
294.8
306.5
318.4
1022.7
1023.2
1023.7
1024.2
1024.7
1019.7
1020.2
1020.7
1021.2
1021.7
1022.2
1016.7
1017.2
1017.7
1018.2
1018.7
1019.2
1013.7
1014.2
1014.7
1015.2
1015.7
1016.2
1010.7
1011.2
1011.7
1012.2
1012.7
1013.2
1010.2
145.2
152.0
159.0
166.9
107.9
113.7
119.7
125.8
132.1
138.6
76.6
81.3
86.3
91.4
96.8
102.3
51.3
55.2
59.1
63.3
67.5
72.0
30.8
34.0
37.2
40.6
44.1
47.6
22.0
24.8
27.8
1004.2
1004.7
1001.2
1001.7
1002.2
1002.7
1003.2
1003.7
998.2
998.7
999.2
999.7
1000.2
1000.7
995.2
995.7
996.2
996.7
997.2
997.7
992.2
992.7
993.2
993.7
994.2
994.7
989.7
990.2
990.7
991.2
991.7
STOKE-107LITTLE YADKIN RIVER WS DAM #29 OCTOBER 27, 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 #29 - STOKE-107 - Watershed Boundary
Legend
Longest FlowPath
Watershed Bound ary
0 0.25 0.50.125 Miles
Date: 10 /5/2020
/Drainage Area: 0.43 Square MilesFlowpath Length: 1.2 miles

APPENDIX B

Lag Time Calculation
Dam No:STOKE-107 Date:10/26/2020
Dam Name:Little Yadkin River WS Dam #29 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 =72.1788 from NRCS CN Calculation
S =3.85448 in.
Mean Basin Slope Y =9.66623 %from Watershed Mean slope
Longest Flowpath:1.19 mi.from Watershed Longest flowpath
Flow Length l =6283.2 ft.
Time of Concentration Tc =0.93 Hours Tc =56 mins
Lag Time TL =0.56 Hours TL =34 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 30116 100 3011563 8378 100 837810 155275 100 15527525
Developed, Open Space Open space - Fair condition (grass cover 50% to 75%)0 49 0 1087194 69 75016388 238314 79 18826784 100490 84 8441177
Developed, Low Intensity Residential districts by average lot size: 1/3 acre 0 57 0 241114 72 17360198 584 81 47329 0 86 0
Developed, Medium Intensity Residential districts by average lot size: 1/8 acre 0 77 0 36034 85 3062881 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 2184926 60 131095550 1022286 73 74626865 871557 79 68852990
Evergreen Forest Woods - Fair 0 36 0 431402 60 25884123 427899 73 31236639 184143 79 14547314
Mixed Forest Woods - Fair 0 36 0 350458 60 21027457 135892 73 9920085 179955 79 14216482
Shrub/Scrub Brush - Fair 0 35 0 12798 56 716666 15026 70 1051850 0 77 0
Herbaceuous Herbaceuous - Fair 0 60 0 699283 71 49649106 658686 81 53353528 17788 89 1583166
Hay/Pasture Pasture - Fair 0 49 0 1138327 69 78544550 582375 79 46007617 106065 84 8909477
Cultivated Crops Row crops (SR + CR) - Poor 0 71 0 338793 80 27103459 464956 87 40451198 282482 90 25423338
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=72.18
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: 36.328653°
Longitude: -80.346°
Elevation (USGS): 1054.68 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 0.859
10-min:0.589 0.703 0.832 0.921 1.03 1.1 1.17 1.23 1.3 1.35
15-min:0.736 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 4.01
2-hr:1.51 1.84 2.33 2.69 3.18 3.57 3.96 4.37 4.91 5.35
3-hr:1.64 2 2.52 2.92 3.47 3.89 4.33 4.77 5.38 5.86
6-hr:2.03 2.47 3.11 3.62 4.32 4.89 5.48 6.1 6.97 7.68
12-hr:2.49 3.03 3.83 4.47 5.38 6.13 6.94 7.8 9.04 10.1
24-hr:2.98 3.61 4.59 5.39 6.55 7.52 8.57 9.71 11.4 12.8
2-day:3.53 4.26 5.38 6.28 7.55 8.59 9.7 10.9 12.6 14
3-day:3.73 4.51 5.68 6.62 7.96 9.05 10.2 11.5 13.2 14.7
4-day:3.94 4.76 5.98 6.97 8.36 9.51 10.7 12 13.9 15.4
7-day:4.48 5.38 6.66 7.69 9.13 10.3 11.5 12.9 14.7 16.3
10-day:5.12 6.12 7.48 8.57 10.1 11.3 12.5 13.9 15.7 17.2
20-day:6.91 8.21 9.84 11.1 12.9 14.2 15.6 17 18.9 20.4
30-day:8.55 10.1 11.9 13.2 14.9 16.2 17.5 18.8 20.4 21.7
45-day:10.8 12.7 14.7 16.2 18.1 19.6 21 22.3 24.1 25.3
60-day:12.8 15.1 17.2 18.8 20.9 22.4 23.9 25.3 27.1 28.4
Date/time (GMT): Mon Oct 26 19:29:53 2020
pyRunTime: 0.0159139633179

APPENDIX F
Outlet Works Rating Curves

Total outlet discharge from the spillway system
Client:NCEM Date:10/23/2020
Project:Overtopping Study Engineer:T.Carter
Dam:Little Yadkin River WS Dam #29 Dam No.STOKE-107
Reservior Water
Level (RWS)
Elevation (ft)
Primary spillway
discharge (cfs)
Emergency spillway
discharge (cfs)
Total discharge from the
spillway system (cfs)
984.7 0.0 0.0 0.0
985.2 3.6 0.0 3.6
985.7 3.8 0.0 3.8
986.2 3.9 0.0 3.9
986.7 3.9 0.0 3.9
987.2 4.0 0.0 4.0
987.7 4.0 0.0 4.0
988.2 4.0 0.0 4.0
988.7 4.1 0.0 4.1
989.2 4.1 0.0 4.1
989.7 4.2 0.0 4.2
990.2 4.2 0.0 4.2
990.7 4.3 0.0 4.3
991.2 4.3 0.0 4.3
991.7 4.4 0.0 4.4
992.2 4.4 0.0 4.4
992.7 4.5 0.0 4.5
993.2 4.5 0.0 4.5
993.7 4.5 0.0 4.5
994.2 4.6 0.0 4.6
994.7 4.6 0.0 4.6
995.2 4.7 9.5 14.2
995.7 4.7 66.1 70.8
996.2 4.8 156.6 161.4
996.7 4.8 278.0 282.8
997.2 4.8 429.6 434.5
997.7 4.9 611.8 616.7
998.2 4.9 825.2 830.1
998.7 5.0 1070.5 1075.4
999.2 5.0 1348.4 1353.4
999.7 5.0 1659.9 1665.0
1000.2 5.1 2005.9 2010.9
1000.7 5.1 2387.1 2392.2
1001.2 5.2 2804.4 2809.6
1001.7 5.2 3258.8 3264.0
1002.2 5.2 3751.1 3756.3
1002.7 5.3 4282.1 4287.3
1003.2 5.3 4852.6 4857.9
1003.7 5.3 5463.6 5468.9
1004.2 5.4 6115.7 6121.0
1004.7 5.4 6809.7 6815.2
1005.2 5.4 7546.6 7552.0
1005.7 5.5 8327.0 8332.5
1006.2 5.5 9151.7 9157.2
1006.7 5.6 10021.4 10027.0
1007.2 5.6 10937.0 10942.6
1007.7 5.6 11899.1 11904.7
1008.2 5.7 12908.4 12914.0
1008.7 5.7 13965.6 13971.3
1009.2 5.7 15071.5 15077.3
1009.7 5.8 16226.8 16232.6
1010.2 5.8 17432.1 17437.9
0.0
2000.0
4000.0
6000.0
8000.0
10000.0
12000.0
14000.0
16000.0
18000.0
20000.0
980.0 985.0 990.0 995.0 1000.0 1005.0 1010.0 1015.0Total discharge from the spillway system (cfs)Reservior Water Level (RWS) Elevation (ft)
The whole spillway system rating curve (cfs)